I don't really understand what you're asking. Are you saying that you're going to go the traditional route, and if the party is fighting bandits, they use the Bandit stat block, or you describe an enemy, but you use whatever stat block looks fun/ is the right CR?
Using the Monster Manual definitely won't lead to you failing on encounters, assuming you use the rules to make balanced encounters.
A 'campaign' is more than just the encounters. So if what you're asking is can a DM wing it by randomly selecting monster stat blocks from the Monster Manual - yes absolutely.
However, a 'campaign' traditionally is a series of adventures that occur over a long period of time. Largely this sort of draws inspiration from the wargaming origins that D&D has. You've got the individual skirmishes that occur within the larger campaign. Effectively the difference between a battle and a war. I genuinely think that most DMs tend to build their campaigns and use stat blocks from Monster Manual, if they do any modification it might be to reskin the monsters, or alter a stat and/or ability here and there. That is after all the purpose of Monster Manual/Bestiaries in most TTRPGs.
So, from your title I read can you just use pre-existing monsters in your self-developed campaign? Yes, absolutely. It's what most people seem to do. In fact from what I've seen its when people try to homebrew their own monsters from scratch that they often go wrong.
Now for a short warning, if you're intending to take a 'wing it' approach to your entire world, setting, and the individual quest(s) that the adventuring party will travel through that's more difficult to do. You'll need to be great at taking and referring to notes. You don't want to be improvising the answer to a question and then contradict yourself six sessions later because you forgot the throwaway thing that a random player asked. So to prevent this if you are going to do a 'make it up as you go along' approach, then you do need to have a pen and paper handy to note down the elements of world building, or lore drops, or even just NPCs names. That way you can refer back to them. I like an A5 ring binder for this purpose, though the 'personal' sized ones are really good too. You can jot down your notes quickly on the loose sheets of paper then organise them with divider tabs in the ring binder later for easier review of your notes. Other people like session based notes, though this does require you knowing in which session you dropped that NPC's name, or the piece of lore.
Incidentally, even if you run games online I'd still highly advise paper and pen note-taking for DMs. The ability to write, organise and review is actually faster in the vast majority of use cases. Yes, people love the find or search function of note-taking apps, but when you cannot remember the name of the NPC being able to flip in half a second to the NPCs page of a physical ring binder is far quicker that even just typing 'NPCs'. I know many who swear by digital who disagree, however as someone actively researching this and who is familiar with revision and education skills I can tell you those people are incorrect from the results that are currently being seen.
It's perfectly fine to stay close to the monster manual and not homebrew your stats. Getting reliable homebrew takes play tests and feedback and time, which can be hard to secure. Skinning exists and is in fact one of the most powerful weapons in your arsenal. However, I find that I enjoy having several sources to draw upon and to have the flexibility to homebrew something if I need to. I myself usually keep to Flee, Mortals! by MCDM (I recommend) and occasionally use stats from some of my other books. I would say that it's okay to use some homebrew in your game for the flexibility and surprise it brings, but it's perfectly fine to just stay away.
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He doesn't have much besides the skin on his bones. Me: I'll take the skin on his bones, then.
"You see a gigantic, monstrous praying mantis burst from out of the ground. It sprays a stream of acid from it's mouth at one soldier, dissolving him instantly, then it turns and chomps another soldier in half with it's- "
You do you. No one knows your capabilities better than you. Some people are plotters and planners, down to every details. And some can wing it and make it amazing.
Go to work. Send your kids to school. Follow fashion. Act normal. Walk on the pavement. Watch TV. Save for old age. Obey the law. Now repeat after me. "YOU MUST CONSTRUCT ADDITIONAL PYLONS"
You are a fool to Start DMing, Just like everyone replying to your posting here. The Stress, The Challenges, The Party destroying any resemblance of any idea you may have. That being said us DM's wouldn't have it any other way. That is why we do it.
You are not going to fail as long as you remember that it is your players interacting with your plot. Let them have fun, Let them challenge you as much as you challenge them, let them make your ideas difficult. Other than that, the rest is just semantics. If everyone at the table is having fun then you are doing a great job. There is no other reason to play DnD. Make shit up on the fly, plan a thing, improvise a thing and create a world that your players will want to return to every session.
Do not worry about it. You will do great.
May your adventures be epic and your rewards be vast.
my plan is to take pre-existing stats and just apply them to whatever encounter I want it. is this foolish beginners optimism, am I going to fail?
I don't really understand what you're asking. Are you saying that you're going to go the traditional route, and if the party is fighting bandits, they use the Bandit stat block, or you describe an enemy, but you use whatever stat block looks fun/ is the right CR?
Using the Monster Manual definitely won't lead to you failing on encounters, assuming you use the rules to make balanced encounters.
A 'campaign' is more than just the encounters. So if what you're asking is can a DM wing it by randomly selecting monster stat blocks from the Monster Manual - yes absolutely.
However, a 'campaign' traditionally is a series of adventures that occur over a long period of time. Largely this sort of draws inspiration from the wargaming origins that D&D has. You've got the individual skirmishes that occur within the larger campaign. Effectively the difference between a battle and a war. I genuinely think that most DMs tend to build their campaigns and use stat blocks from Monster Manual, if they do any modification it might be to reskin the monsters, or alter a stat and/or ability here and there. That is after all the purpose of Monster Manual/Bestiaries in most TTRPGs.
So, from your title I read can you just use pre-existing monsters in your self-developed campaign? Yes, absolutely. It's what most people seem to do. In fact from what I've seen its when people try to homebrew their own monsters from scratch that they often go wrong.
Now for a short warning, if you're intending to take a 'wing it' approach to your entire world, setting, and the individual quest(s) that the adventuring party will travel through that's more difficult to do. You'll need to be great at taking and referring to notes. You don't want to be improvising the answer to a question and then contradict yourself six sessions later because you forgot the throwaway thing that a random player asked. So to prevent this if you are going to do a 'make it up as you go along' approach, then you do need to have a pen and paper handy to note down the elements of world building, or lore drops, or even just NPCs names. That way you can refer back to them. I like an A5 ring binder for this purpose, though the 'personal' sized ones are really good too. You can jot down your notes quickly on the loose sheets of paper then organise them with divider tabs in the ring binder later for easier review of your notes. Other people like session based notes, though this does require you knowing in which session you dropped that NPC's name, or the piece of lore.
Incidentally, even if you run games online I'd still highly advise paper and pen note-taking for DMs. The ability to write, organise and review is actually faster in the vast majority of use cases. Yes, people love the find or search function of note-taking apps, but when you cannot remember the name of the NPC being able to flip in half a second to the NPCs page of a physical ring binder is far quicker that even just typing 'NPCs'. I know many who swear by digital who disagree, however as someone actively researching this and who is familiar with revision and education skills I can tell you those people are incorrect from the results that are currently being seen.
DM session planning template - My version of maps for 'Lost Mine of Phandelver' - Send your party to The Circus - Other DM Resources - Maps, Tokens, Quests - 'Better' Player Character Injury Tables?
Actor, Writer, Director & Teacher by day - GM/DM in my off hours.
It's perfectly fine to stay close to the monster manual and not homebrew your stats. Getting reliable homebrew takes play tests and feedback and time, which can be hard to secure. Skinning exists and is in fact one of the most powerful weapons in your arsenal. However, I find that I enjoy having several sources to draw upon and to have the flexibility to homebrew something if I need to. I myself usually keep to Flee, Mortals! by MCDM (I recommend) and occasionally use stats from some of my other books. I would say that it's okay to use some homebrew in your game for the flexibility and surprise it brings, but it's perfectly fine to just stay away.
He doesn't have much besides the skin on his bones. Me: I'll take the skin on his bones, then.
"You see a gigantic, monstrous praying mantis burst from out of the ground. It sprays a stream of acid from it's mouth at one soldier, dissolving him instantly, then it turns and chomps another soldier in half with it's- "
"When are we gonna take a snack break?"
You do you. No one knows your capabilities better than you. Some people are plotters and planners, down to every details. And some can wing it and make it amazing.
I would try a one shot first.
Good luck.
Anzio Faro. Protector Aasimar light cleric. Lvl 18.
Viktor Gavriil. White dragonborn grave cleric. Lvl 20.
Ikram Sahir ibn-Malik al-Sayyid Ra'ad. Brass dragonborn draconic sorcerer Lvl 9. Fire elemental devil.
Wrangler of cats.
Now I feel overwhelmed
Go to work. Send your kids to school. Follow fashion. Act normal. Walk on the pavement. Watch TV. Save for old age. Obey the law. Now repeat after me. "YOU MUST CONSTRUCT ADDITIONAL PYLONS"
You are a fool to Start DMing, Just like everyone replying to your posting here. The Stress, The Challenges, The Party destroying any resemblance of any idea you may have. That being said us DM's wouldn't have it any other way. That is why we do it.
You are not going to fail as long as you remember that it is your players interacting with your plot. Let them have fun, Let them challenge you as much as you challenge them, let them make your ideas difficult. Other than that, the rest is just semantics. If everyone at the table is having fun then you are doing a great job. There is no other reason to play DnD. Make shit up on the fly, plan a thing, improvise a thing and create a world that your players will want to return to every session.
Do not worry about it. You will do great.
May your adventures be epic and your rewards be vast.
Have fun my friend
If just going by the MM, you could do just fine, as long as you only look at the tarrasque in case of an emergency.