I'm fairly new to DM'ing, having only DM'ed a few one-shots. I'm currently trying to come up with a high CR creature to make into my BBEG. My thoughts are a Dracolich that appears throughout history building an empire and killing Gods. Wanting to make it around a CR 60 or 70. Players will end up around the equivalent of level 50. Thoughts and suggestions are greatly appreciated.
You're trying to run before you can crawl at this point.
Max character level is 20 in 5e.
CR30 is the maxium from official sources that I know of.
If you've only DM'd a few one shots, you don't yet have either a dedicated group that will reach that level (reaching level 20 typically takes about 4-5 years of real time, assuming playing 10-12 hours every 4 weeks) and don't have the DM'ing experience to be planning ultra-homebrew-endgame yet (or a need to).
You may have been a player before, you don't say, but I don't think I could easily explain the important aspects of designing a boss for high level play to someone who has not taken characters to at least level 12.
You're basically trying to plan something that won't matter for years of real time (if you ever reach that point).
Start small. Set reasonable, achievable objectives for your game.
Plan a small village and its NPCs. Create a local environment, and decide what's causing the problems in the areas.
Run 3-4 games where the PCs do some stuff in the village's surroundings. The clues they pick up should lead them to realise what is causing the problem.
Spend around 3 sessions where the PCs deal with the issue.
For me, this would take the PCs to reach level 5 at the end of that session. The PCs can then move on to a new area in the campaign world.
If you want to indicate that the local cult or whatever it is worships a Dracolich, you can start that off during this sequence of events. But you won't need stats for it for literal years of gameplay, since if it showed up, the heroes probably just die anyway.
I have been a player for over 15 years. I have a group I run with. We have been together for 3 years. As for tips on the Dracolich, it will appear to the players through-out the campaign. Their first meeting will be the Dracolich giving them their first quest. And the goal of this is for the players to go beyond level 20. Milestone leveling. As for my inexperience as a DM, I'm typically quiet and reserved since I have anxiety and PTSD. This is my way of trying to step out of my comfort zone. But I have the mind and dedication for my idea. I just need help with the Dracolich.
I will be using other sources to take my players beyond level 20. I have a source that takes players to level 30. Then Godsfall. 5E compatible. Makes characters into Gods.
Okay, I've poked around Godsfall a little bit; looks interesting! With the Divinities system from that book, I can see how your players might be able to take on a CR 70 monster at full power. Are you looking for advice about how to build the Dracolich mechanically, or are you looking for advice about how to work it into the story, or both?
I'm looking for help building the Dracolich. HP, AC, Spells. Things like that. I have the story. One thing I'm keeping in mind is that this thing has existed for over 10,000 years. So it has vast knowledge that it has learned over that time.
It's going to be hard for anyone to advise you on how to build the Dracolich from a stats perspective, since there is no way to judge what a level 50 character would be like. Do they have level 25 spells? What are those spells? Or do they multiclass like 20 Wizard/20 Fighter/10 Rogue? Either way, there's no way anyone can judge that. I guess I can say that for my 6 level 16 PCs, 2000hp is not too much. So I guess like... 50k hp.
In terms of making a Dracolich as an enemy, if you're intending to run a campaign all the way to level 50, you're going to need more than one BBEG. I'd think of the campaign in terms of "Acts." Act 1 from level 1-10. Act 2 from 11-20 and so on. Each should be a self contained story, effectively, though you can seed in the bigger enemies. The PCs can't be up against one enemy the entire time and this also allows for satisfying ending points if players have to leave the game. A Dracolich isn't really a match for characters of the power levels you're aiming for: by the time PCs are level 20, they are practically god-like (which is why so very, very few campaigns even go into the higher levels). So from 20 onwards, they'll need to be up against demi-gods, demon princes, and eventually the gods themselves.
Maybe the dracolich could be the avatar form of an ancient god, bound to a dead dragon's body? It'll need to be ultra-epic, since your PCs will have all the tools they need to beat any mortal foe before they're even halfway.
Story-wise, the Dracolich is a corrupted Ancient Gold Dragon. Nobody but the Gods knows why or how this Gold became corrupted (An evil God killed this Dragons kin and it became so obsessed with revenge that it turned evil and became a Dracolich to become powerful enough to destroy the whole pantheon. The group will have to find a way to either turn it or kill it. Both could be done. Also, I will have Arcs. The Arc before the final Arc, the group will confront Tiamat and Bahamut to get info and potentially fight.
I'd second the comments that we simply don't have enough information. I'd also suggest that you don't need detailed info on what the creature can do now. You only need it from a role playing perspective since until the characters reach these god-like levels, they will be ineffective against this "lich".
I'd also add that likely 99% of the people on these forums would be considered a "new" DM when trying to design an NPC that is so far beyond the capabilities of the 5e system that it requires home brew (with which very few of us are familiar) to represent the abilities of PCs and NPCs at that level.
For example - I think you don't want a "Dracolich" ... you want an immortal undead dragon god with spellcasting abilities. A Dracolich (as an existing creature) is so far underpowered for what you are imagining that I'd just toss out any preconceived ideas that might come along with calling it a "Dracolich".
Typical abilities:
1) Creature can assume any form desired at will (action). It can appear as any humanoid, any dragon etc ... basically Level 9 Shapechange at will. This creature isn't a "Dracolich" it is a "god" or equivalent to one.
2) Is it limited by spell slots? Do spells go beyond level 9? Can this creature cast Wish at will?
3) Is this creature Omniscient? Does it have the ability to see/hear other places? Are there any limitations on what it can choose to see and hear and can it observe/commune with multiple locations/creatures at once?
4) Does this creature have a priesthood and followers? Does it run an organization of cultists and acolytes? Does it gather power from followers and is attacking its followers a method of weakening the creatures god-like potential?
You are planning a campaign at an extreme level but it is up to you to define exactly how extreme it will be. In 5e, gods are immortal, they can't be killed, any stat blocks available are simply avatars that can be defeated but the god itself can not be (at least that is my understanding). You will likely be changing these precepts.
Anyway
Hit points: 2000, 50k, 100k, 1,000,000? This depends on how much damage your level 50 PCs are capable of ... the more they can do, the higher the creatures hit points need to be.
AC: 25, 30, 50, 100? This depends on how large the to hit bonuses of the PCs will get by level 50? Are you still using 5e bounded accuracy? If the PCs have +20 to hit then a 30AC means they hit about 1/2 the time. If that is how often you want them to hit then that decides how much AC it should have.
Damage/condition resists and immunities .. can it be affected by anything? Stun? Prone? Blindness? Is it resistant to all damage (all this does is effectively double its hit points so it isn't very interesting)? Is it immune to bludgeoning/piercing/slashing damage - this makes martial/weapon attacks much weaker.
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Basically, the NPC can have ANY numbers you like for its stat block BUT you won't really know (or need) those numbers until the NPC comes into a direct conflict with the PC where those numbers matter. Until then, the REALLY important info, is what are the goals of this creature, how do they plan to achieve those goals, can they manipulate the PCs to unwittingly help with those goals? If I was writing this campaign - I would know the BBEG and what they want to do and only create a stat block when I actually need it. Starting with a stat block at the beginning really doesn't help since you might find you need to change it anyway to adapt to events in the adventure - so why bother fixing it at this point in the first place.
Since it's god-like if not an actual god, I'd have it something that you can't straight up fight. The PCs might need an artifact, that takes a full arc to craft/recover/put back together, that can destroy this thing.
You'll need a reason that the god-like death dragon doesn't just intervene and one-shot the PCs. They'll be famous long before they ever get near taking a shot at it. Why doesn't it just teleport in and step on them? For the power level it's going to be at, it will need to be able to cast multiple 9th level spells each turn - probably at will. So what's holding it back?
What's holding it back is that it treats humanoids like toys and plays with them. It will randomly appear throughout the campaign and either try to corrupt them or kill one or 2 possibly. Somebody WILL die. It's all a part of the story. The first death will be where they find out about it. And the campaign will take course over years in game. Downtime, different arcs, side quests.
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
What's holding it back is that it treats humanoids like toys and plays with them. It will randomly appear throughout the campaign and either try to corrupt them or kill one or 2 possibly. Somebody WILL die. It's all a part of the story. The first death will be where they find out about it. And the campaign will take course over years in game. Downtime, different arcs, side quests.
I strongly recommend not deciding that a PC will die. Not sure if what you were inferring is that you intend it as a plot element, but if so, my advice is: don't do that. The DM should present challenges, and even if they're very tough ones, the idea of a god-power level being showing up and there's nothing one can do to stop it is something I wouldn't tolerate, as a player. That would be me out of the campaign. Aiming for corruption, now that sounds like fun - but when you say it's all a part of the story, remember that the players are making the story with you.
The idea that "the BBEG just doesn't want to wipe us out" is not a strong limiter. It means there's no way the players can control whether they live or die, within the parameters you've said. This mega-dragon-god can show up and just off someone if they want to? That's you writing a story. It's not a game.
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
Oh yeah, I know that feeling for sure. Fortunately for you, most of the most powerful monsters in the game are already dragons, and Dracolich is a template rather than a distinct creature. This gives you a very easy starting point: take a Gold Greatwyrm, from Fizban's Treasury of Dragons and apply the Dracolich template. But that's only going to get you to about CR29, so you'll want to add some additional layers. Obviously your lich-dragon-god will get all the "Dragons as inherent spellcasters" options, and I think Damage Absorption (heals when subjected to a damage type it's immune to) and Regeneration (comes back to life in 1d10 days if killed but not destroyed at the source of its power) also fit it very well. Those options could potentially bring it up to CR 30. Then we can start having fun.
If this thing is as ancient and powerful as you say, it's had time to learn some things. Obviously you can give it additional spells, but that's thinking too small. Give it a bunch of other creatures' Actions. Just start looking through the highest CR monsters you have access to and start lifting things you think are cool. Give it the Greater Star Spawn Emissary's Mind Cloud, give it Geryon's Hateful Restraints, give it the Marut's Blazing Edict, just load this thing up with as many cool abilities as you think you'll reasonably use in a three-stage combat. Parcel these abilities out between your Great Dracolich's base form, Mythic form, and third-phase. Also, it should have a third phase; you can model that off the way Mythic monsters work, just add another layer to it.
Now there's one more thing you can do. By this phase of the campaign, your players' Divinities will have fully manifested and they'll be really feeling themselves. So give the lich a Divinity, too. You don't necessarily need to make this boss a full PC (and you probably shouldn't, PCs are a real pain to run as monsters) but pick a few trademark abilities from a fitting Divinity progression (or more than one!) and let the Great Dracolich reveal those over the course of the fight. Trust me, players who like intense fights love it when a monster pulls out tricks normally reserved for PCs.
Now... will all of this actually be enough? Will it even be playable? I genuinely have no idea. You're kind of breaking the fundamental design conceit of 5e by even attempting this. This whole edition is based on Bounded Accuracy, which is basically the idea that all PCs stats fit within a predictable box so that it's easier to design encounters and recycle weak monsters. You are absolutely shattering that box, so while we can throw out ideas for things you could do, there's genuinely no way to predict if any of it will be useful by the time you get to this fight. This is not me saying "Don't design this boss right now", but it is me saying "Be ready to totally re-design this boss several times as you get a feel for what your busted PCs can actually do."
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
Oh yeah, I know that feeling for sure. Fortunately for you, most of the most powerful monsters in the game are already dragons, and Dracolich is a template rather than a distinct creature. This gives you a very easy starting point: take a Gold Greatwyrm, from Fizban's Treasury of Dragons and apply the Dracolich template. But that's only going to get you to about CR29, so you'll want to add some additional layers. Obviously your lich-dragon-god will get all the "Dragons as inherent spellcasters" options, and I think Damage Absorption (heals when subjected to a damage type it's immune to) and Regeneration (comes back to life in 1d10 days if killed but not destroyed at the source of its power) also fit it very well. Those options could potentially bring it up to CR 30. Then we can start having fun.
If this thing is as ancient and powerful as you say, it's had time to learn some things. Obviously you can give it additional spells, but that's thinking too small. Give it a bunch of other creatures' Actions. Just start looking through the highest CR monsters you have access to and start lifting things you think are cool. Give it the Greater Star Spawn Emissary's Mind Cloud, give it Geryon's Hateful Restraints, give it the Marut's Blazing Edict, just load this thing up with as many cool abilities as you think you'll reasonably use in a three-stage combat. Parcel these abilities out between your Great Dracolich's base form, Mythic form, and third-phase. Also, it should have a third phase; you can model that off the way Mythic monsters work, just add another layer to it.
Now there's one more thing you can do. By this phase of the campaign, your players' Divinities will have fully manifested and they'll be really feeling themselves. So give the lich a Divinity, too. You don't necessarily need to make this boss a full PC (and you probably shouldn't, PCs are a real pain to run as monsters) but pick a few trademark abilities from a fitting Divinity progression (or more than one!) and let the Great Dracolich reveal those over the course of the fight. Trust me, players who like intense fights love it when a monster pulls out tricks normally reserved for PCs.
Now... will all of this actually be enough? Will it even be playable? I genuinely have no idea. You're kind of breaking the fundamental design conceit of 5e by even attempting this. This whole edition is based on Bounded Accuracy, which is basically the idea that all PCs stats fit within a predictable box so that it's easier to design encounters and recycle weak monsters. You are absolutely shattering that box, so while we can throw out ideas for things you could do, there's genuinely no way to predict if any of it will be useful by the time you get to this fight. This is not me saying "Don't design this boss right now", but it is me saying "Be ready to totally re-design this boss several times as you get a feel for what your busted PCs can actually do."
Thank you. You have been the most helpful. While I was being told "You don't need to design this right now', you were giving me ideas. Thank you very much. I wish there was a way I could chat with you and bounce some ideas to you. Also to get some info on the things you mentioned. I don't have those sources.
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
Oh yeah, I know that feeling for sure. Fortunately for you, most of the most powerful monsters in the game are already dragons, and Dracolich is a template rather than a distinct creature. This gives you a very easy starting point: take a Gold Greatwyrm, from Fizban's Treasury of Dragons and apply the Dracolich template. But that's only going to get you to about CR29, so you'll want to add some additional layers. Obviously your lich-dragon-god will get all the "Dragons as inherent spellcasters" options, and I think Damage Absorption (heals when subjected to a damage type it's immune to) and Regeneration (comes back to life in 1d10 days if killed but not destroyed at the source of its power) also fit it very well. Those options could potentially bring it up to CR 30. Then we can start having fun.
If this thing is as ancient and powerful as you say, it's had time to learn some things. Obviously you can give it additional spells, but that's thinking too small. Give it a bunch of other creatures' Actions. Just start looking through the highest CR monsters you have access to and start lifting things you think are cool. Give it the Greater Star Spawn Emissary's Mind Cloud, give it Geryon's Hateful Restraints, give it the Marut's Blazing Edict, just load this thing up with as many cool abilities as you think you'll reasonably use in a three-stage combat. Parcel these abilities out between your Great Dracolich's base form, Mythic form, and third-phase. Also, it should have a third phase; you can model that off the way Mythic monsters work, just add another layer to it.
Now there's one more thing you can do. By this phase of the campaign, your players' Divinities will have fully manifested and they'll be really feeling themselves. So give the lich a Divinity, too. You don't necessarily need to make this boss a full PC (and you probably shouldn't, PCs are a real pain to run as monsters) but pick a few trademark abilities from a fitting Divinity progression (or more than one!) and let the Great Dracolich reveal those over the course of the fight. Trust me, players who like intense fights love it when a monster pulls out tricks normally reserved for PCs.
Now... will all of this actually be enough? Will it even be playable? I genuinely have no idea. You're kind of breaking the fundamental design conceit of 5e by even attempting this. This whole edition is based on Bounded Accuracy, which is basically the idea that all PCs stats fit within a predictable box so that it's easier to design encounters and recycle weak monsters. You are absolutely shattering that box, so while we can throw out ideas for things you could do, there's genuinely no way to predict if any of it will be useful by the time you get to this fight. This is not me saying "Don't design this boss right now", but it is me saying "Be ready to totally re-design this boss several times as you get a feel for what your busted PCs can actually do."
Thank you. You have been the most helpful. While I was being told "You don't need to design this right now', you were giving me ideas. Thank you very much. I wish there was a way I could chat with you and bounce some ideas to you. Also to get some info on the things you mentioned. I don't have those sources.
It is possible to calculate the stats that will make for a good fight. You just need to know:
Party size
Average party damage output per turn at (homebrew) level 50 (a rough figure is fine) and the number of turns you want the combat to last (I suggest 9-12 at high level)
Likely healing per turn capabilities of the party at (homebrew) level 50 (rough figure is fine)
Average party hit points at (homebrew) level 50
Any magical items/artifacts the party is likely to have access to
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
Oh yeah, I know that feeling for sure. Fortunately for you, most of the most powerful monsters in the game are already dragons, and Dracolich is a template rather than a distinct creature. This gives you a very easy starting point: take a Gold Greatwyrm, from Fizban's Treasury of Dragons and apply the Dracolich template. But that's only going to get you to about CR29, so you'll want to add some additional layers. Obviously your lich-dragon-god will get all the "Dragons as inherent spellcasters" options, and I think Damage Absorption (heals when subjected to a damage type it's immune to) and Regeneration (comes back to life in 1d10 days if killed but not destroyed at the source of its power) also fit it very well. Those options could potentially bring it up to CR 30. Then we can start having fun.
If this thing is as ancient and powerful as you say, it's had time to learn some things. Obviously you can give it additional spells, but that's thinking too small. Give it a bunch of other creatures' Actions. Just start looking through the highest CR monsters you have access to and start lifting things you think are cool. Give it the Greater Star Spawn Emissary's Mind Cloud, give it Geryon's Hateful Restraints, give it the Marut's Blazing Edict, just load this thing up with as many cool abilities as you think you'll reasonably use in a three-stage combat. Parcel these abilities out between your Great Dracolich's base form, Mythic form, and third-phase. Also, it should have a third phase; you can model that off the way Mythic monsters work, just add another layer to it.
Now there's one more thing you can do. By this phase of the campaign, your players' Divinities will have fully manifested and they'll be really feeling themselves. So give the lich a Divinity, too. You don't necessarily need to make this boss a full PC (and you probably shouldn't, PCs are a real pain to run as monsters) but pick a few trademark abilities from a fitting Divinity progression (or more than one!) and let the Great Dracolich reveal those over the course of the fight. Trust me, players who like intense fights love it when a monster pulls out tricks normally reserved for PCs.
Now... will all of this actually be enough? Will it even be playable? I genuinely have no idea. You're kind of breaking the fundamental design conceit of 5e by even attempting this. This whole edition is based on Bounded Accuracy, which is basically the idea that all PCs stats fit within a predictable box so that it's easier to design encounters and recycle weak monsters. You are absolutely shattering that box, so while we can throw out ideas for things you could do, there's genuinely no way to predict if any of it will be useful by the time you get to this fight. This is not me saying "Don't design this boss right now", but it is me saying "Be ready to totally re-design this boss several times as you get a feel for what your busted PCs can actually do."
Thank you. You have been the most helpful. While I was being told "You don't need to design this right now', you were giving me ideas. Thank you very much. I wish there was a way I could chat with you and bounce some ideas to you. Also to get some info on the things you mentioned. I don't have those sources.
It is possible to calculate the stats that will make for a good fight. You just need to know:
Party size
Average party damage output per turn at (homebrew) level 50 (a rough figure is fine) and the number of turns you want the combat to last (I suggest 9-12 at high level)
Likely healing per turn capabilities of the party at (homebrew) level 50 (rough figure is fine)
Average party hit points at (homebrew) level 50
Any magical items/artifacts the party is likely to have access to
Thank you Sanvael. This will help. You have been helpful as well.
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I'm fairly new to DM'ing, having only DM'ed a few one-shots. I'm currently trying to come up with a high CR creature to make into my BBEG. My thoughts are a Dracolich that appears throughout history building an empire and killing Gods. Wanting to make it around a CR 60 or 70. Players will end up around the equivalent of level 50. Thoughts and suggestions are greatly appreciated.
Could you elaborate what you mean by "Players will end up around the equivalent of level 50"?
You're trying to run before you can crawl at this point.
You're basically trying to plan something that won't matter for years of real time (if you ever reach that point).
I have been a player for over 15 years. I have a group I run with. We have been together for 3 years. As for tips on the Dracolich, it will appear to the players through-out the campaign. Their first meeting will be the Dracolich giving them their first quest. And the goal of this is for the players to go beyond level 20. Milestone leveling. As for my inexperience as a DM, I'm typically quiet and reserved since I have anxiety and PTSD. This is my way of trying to step out of my comfort zone. But I have the mind and dedication for my idea. I just need help with the Dracolich.
I will be using other sources to take my players beyond level 20. I have a source that takes players to level 30. Then Godsfall. 5E compatible. Makes characters into Gods.
Okay, I've poked around Godsfall a little bit; looks interesting! With the Divinities system from that book, I can see how your players might be able to take on a CR 70 monster at full power. Are you looking for advice about how to build the Dracolich mechanically, or are you looking for advice about how to work it into the story, or both?
I'm looking for help building the Dracolich. HP, AC, Spells. Things like that. I have the story. One thing I'm keeping in mind is that this thing has existed for over 10,000 years. So it has vast knowledge that it has learned over that time.
It's going to be hard for anyone to advise you on how to build the Dracolich from a stats perspective, since there is no way to judge what a level 50 character would be like. Do they have level 25 spells? What are those spells? Or do they multiclass like 20 Wizard/20 Fighter/10 Rogue? Either way, there's no way anyone can judge that. I guess I can say that for my 6 level 16 PCs, 2000hp is not too much. So I guess like... 50k hp.
In terms of making a Dracolich as an enemy, if you're intending to run a campaign all the way to level 50, you're going to need more than one BBEG. I'd think of the campaign in terms of "Acts." Act 1 from level 1-10. Act 2 from 11-20 and so on. Each should be a self contained story, effectively, though you can seed in the bigger enemies. The PCs can't be up against one enemy the entire time and this also allows for satisfying ending points if players have to leave the game. A Dracolich isn't really a match for characters of the power levels you're aiming for: by the time PCs are level 20, they are practically god-like (which is why so very, very few campaigns even go into the higher levels). So from 20 onwards, they'll need to be up against demi-gods, demon princes, and eventually the gods themselves.
Maybe the dracolich could be the avatar form of an ancient god, bound to a dead dragon's body? It'll need to be ultra-epic, since your PCs will have all the tools they need to beat any mortal foe before they're even halfway.
Story-wise, the Dracolich is a corrupted Ancient Gold Dragon. Nobody but the Gods knows why or how this Gold became corrupted (An evil God killed this Dragons kin and it became so obsessed with revenge that it turned evil and became a Dracolich to become powerful enough to destroy the whole pantheon. The group will have to find a way to either turn it or kill it. Both could be done. Also, I will have Arcs. The Arc before the final Arc, the group will confront Tiamat and Bahamut to get info and potentially fight.
I'd second the comments that we simply don't have enough information. I'd also suggest that you don't need detailed info on what the creature can do now. You only need it from a role playing perspective since until the characters reach these god-like levels, they will be ineffective against this "lich".
I'd also add that likely 99% of the people on these forums would be considered a "new" DM when trying to design an NPC that is so far beyond the capabilities of the 5e system that it requires home brew (with which very few of us are familiar) to represent the abilities of PCs and NPCs at that level.
For example - I think you don't want a "Dracolich" ... you want an immortal undead dragon god with spellcasting abilities. A Dracolich (as an existing creature) is so far underpowered for what you are imagining that I'd just toss out any preconceived ideas that might come along with calling it a "Dracolich".
Typical abilities:
1) Creature can assume any form desired at will (action). It can appear as any humanoid, any dragon etc ... basically Level 9 Shapechange at will. This creature isn't a "Dracolich" it is a "god" or equivalent to one.
2) Is it limited by spell slots? Do spells go beyond level 9? Can this creature cast Wish at will?
3) Is this creature Omniscient? Does it have the ability to see/hear other places? Are there any limitations on what it can choose to see and hear and can it observe/commune with multiple locations/creatures at once?
4) Does this creature have a priesthood and followers? Does it run an organization of cultists and acolytes? Does it gather power from followers and is attacking its followers a method of weakening the creatures god-like potential?
You are planning a campaign at an extreme level but it is up to you to define exactly how extreme it will be. In 5e, gods are immortal, they can't be killed, any stat blocks available are simply avatars that can be defeated but the god itself can not be (at least that is my understanding). You will likely be changing these precepts.
Anyway
Hit points: 2000, 50k, 100k, 1,000,000? This depends on how much damage your level 50 PCs are capable of ... the more they can do, the higher the creatures hit points need to be.
AC: 25, 30, 50, 100? This depends on how large the to hit bonuses of the PCs will get by level 50? Are you still using 5e bounded accuracy? If the PCs have +20 to hit then a 30AC means they hit about 1/2 the time. If that is how often you want them to hit then that decides how much AC it should have.
Damage/condition resists and immunities .. can it be affected by anything? Stun? Prone? Blindness? Is it resistant to all damage (all this does is effectively double its hit points so it isn't very interesting)? Is it immune to bludgeoning/piercing/slashing damage - this makes martial/weapon attacks much weaker.
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Basically, the NPC can have ANY numbers you like for its stat block BUT you won't really know (or need) those numbers until the NPC comes into a direct conflict with the PC where those numbers matter. Until then, the REALLY important info, is what are the goals of this creature, how do they plan to achieve those goals, can they manipulate the PCs to unwittingly help with those goals? If I was writing this campaign - I would know the BBEG and what they want to do and only create a stat block when I actually need it. Starting with a stat block at the beginning really doesn't help since you might find you need to change it anyway to adapt to events in the adventure - so why bother fixing it at this point in the first place.
Since it's god-like if not an actual god, I'd have it something that you can't straight up fight. The PCs might need an artifact, that takes a full arc to craft/recover/put back together, that can destroy this thing.
You'll need a reason that the god-like death dragon doesn't just intervene and one-shot the PCs. They'll be famous long before they ever get near taking a shot at it. Why doesn't it just teleport in and step on them? For the power level it's going to be at, it will need to be able to cast multiple 9th level spells each turn - probably at will. So what's holding it back?
What's holding it back is that it treats humanoids like toys and plays with them. It will randomly appear throughout the campaign and either try to corrupt them or kill one or 2 possibly. Somebody WILL die. It's all a part of the story. The first death will be where they find out about it. And the campaign will take course over years in game. Downtime, different arcs, side quests.
The reason that I'm getting this thing fleshed out is bc that is how my brain works. I get hyper-focused and can't focus on anything else until that's done. And I do like the idea of an undead dragon god.
I strongly recommend not deciding that a PC will die. Not sure if what you were inferring is that you intend it as a plot element, but if so, my advice is: don't do that. The DM should present challenges, and even if they're very tough ones, the idea of a god-power level being showing up and there's nothing one can do to stop it is something I wouldn't tolerate, as a player. That would be me out of the campaign. Aiming for corruption, now that sounds like fun - but when you say it's all a part of the story, remember that the players are making the story with you.
The idea that "the BBEG just doesn't want to wipe us out" is not a strong limiter. It means there's no way the players can control whether they live or die, within the parameters you've said. This mega-dragon-god can show up and just off someone if they want to? That's you writing a story. It's not a game.
I see your point.
Oh yeah, I know that feeling for sure. Fortunately for you, most of the most powerful monsters in the game are already dragons, and Dracolich is a template rather than a distinct creature. This gives you a very easy starting point: take a Gold Greatwyrm, from Fizban's Treasury of Dragons and apply the Dracolich template. But that's only going to get you to about CR29, so you'll want to add some additional layers. Obviously your lich-dragon-god will get all the "Dragons as inherent spellcasters" options, and I think Damage Absorption (heals when subjected to a damage type it's immune to) and Regeneration (comes back to life in 1d10 days if killed but not destroyed at the source of its power) also fit it very well. Those options could potentially bring it up to CR 30. Then we can start having fun.
If this thing is as ancient and powerful as you say, it's had time to learn some things. Obviously you can give it additional spells, but that's thinking too small. Give it a bunch of other creatures' Actions. Just start looking through the highest CR monsters you have access to and start lifting things you think are cool. Give it the Greater Star Spawn Emissary's Mind Cloud, give it Geryon's Hateful Restraints, give it the Marut's Blazing Edict, just load this thing up with as many cool abilities as you think you'll reasonably use in a three-stage combat. Parcel these abilities out between your Great Dracolich's base form, Mythic form, and third-phase. Also, it should have a third phase; you can model that off the way Mythic monsters work, just add another layer to it.
Now there's one more thing you can do. By this phase of the campaign, your players' Divinities will have fully manifested and they'll be really feeling themselves. So give the lich a Divinity, too. You don't necessarily need to make this boss a full PC (and you probably shouldn't, PCs are a real pain to run as monsters) but pick a few trademark abilities from a fitting Divinity progression (or more than one!) and let the Great Dracolich reveal those over the course of the fight. Trust me, players who like intense fights love it when a monster pulls out tricks normally reserved for PCs.
Now... will all of this actually be enough? Will it even be playable? I genuinely have no idea. You're kind of breaking the fundamental design conceit of 5e by even attempting this. This whole edition is based on Bounded Accuracy, which is basically the idea that all PCs stats fit within a predictable box so that it's easier to design encounters and recycle weak monsters. You are absolutely shattering that box, so while we can throw out ideas for things you could do, there's genuinely no way to predict if any of it will be useful by the time you get to this fight. This is not me saying "Don't design this boss right now", but it is me saying "Be ready to totally re-design this boss several times as you get a feel for what your busted PCs can actually do."
Thank you. You have been the most helpful. While I was being told "You don't need to design this right now', you were giving me ideas. Thank you very much. I wish there was a way I could chat with you and bounce some ideas to you. Also to get some info on the things you mentioned. I don't have those sources.
It is possible to calculate the stats that will make for a good fight. You just need to know:
Thank you Sanvael. This will help. You have been helpful as well.