Oh I hope to DM this someday.  I’d like to think it’s not railroad-y, since the Lich’s backstory is all set thousands of years before the adventurers where even born.  (I mean what good story doesn’t have some epic events happening LONG LONG AGO).

I feel like the Gods are stronger then Archdevils, but not enough to fight them on the Archdevil’s home turf.  And definitely not enough to deal with all of them at once (maybe if all the Gods joined together, but that’s still a prolonged fight and a hard sell).  As to why the God’s don’t just step in themselves and deal with the problem.  They can’t manifest fully onto the prime material plane.  Any being beyond a certain level of power isn’t suppose to be able to pass through the Divine Gate.  So the fact that an Archdevil somehow managed to find a way through, causes some pretty major issues.

I figure once the Gods figured out what was happening / what was about to happen, they decided to cut their losses.  Better to lose a bunch of followers now, then to try and continue the fight and risk not being strong enough to survive the lull/downtime that comes between abandoning one realm and making a new one.

Sure.  Tough fight, so just give up and abandon all of creation.  Make a better world somewhere else.  Very dark setting.  The gods are just a bunch of uncaring refugees without the strength to stand up and fight.   Not something I'd ever want to be in, but some people like a game where all hope is lost, and all you can do is see how long you can hold off death, or just become undead, which is really the same thing, only you end up taking long, long time.  That's why most Liches end up being destroyed in the end.  For all their power, boredom gets them eventually.

In Fifth Edition, liches need to do periodic upkeep on their phylacteries by feeding them souls using the Imprisonment spell, which currently only Wizards and Warlocks can cast. Technically Bards as well. So you'd have to overcome that obstacle. In earlier editions, I don't know that liches had to do that, since the SRD said typical liches were sorcerers or wizards, but other types existed as well.

I see that I did not explain this properly.

 Of course the Gods warn those they can, they offer some individuals the chance to flee with them.  Start over in a new world, live a life free of the conflicts that have shaped your existence thus far.  But I’m not going to make the Gods so powerful they can simply just “save everyone” that is a terrible storyline.  If the Gods were that strong, why haven’t they simply dealt with the issue long before it became an issue?  I’ve yet to find a good story where the gods exist, are all powerful, caring, but yet evil is still a thing that exists and is a danger to people.

I’m not sure why you think the heroes are joining the villains. They are not.  The Heroes simply had to find ways to defeat the villains without access to the divine powers they had previously been using.  I am curious why you think the Gods are weak, and why you say the cleric has failed them?

Edit:  it strikes me that people seem to think that I’m forcing this story onto a PC.  I am not.  This is 100% NPC backstory for one of the BBEGs.

Edit 2:  I should probably also point out that the BBEG is successful in his goal, stops the Archdevil from manifesting onto the plane.  It’s a big battle, large areas of the world are warped/destroyed/or changed forever, but the BBEG/Heroes party comes out victorious.  The Gods who had not fully fled the world, eventually reassert themselves.  And they use their influence (and the fact that very few people survived who know what REALLY HAPPENED) to rewrite history and paint themselves in the best of light.  You know the old “and thus the gods did take pity on the wretched masses.  Thus did they send their divine warriors forth to slay the darkness and beat back to armies of evil”

So let’s break this down.

1) the Archdevils are roughly on par with the gods (as they are in basically ALL the d&d lore).  Like the Archdevils would lose if they went to the Gods domain to fight them, and the Gods would lose if they went to the Hells to fight the Archdevils.

2) The reason the Gods cant do anything, is because they put a giant metaphysical fence between them (and other things of unimaginable power) and the moral realm.  They can’t enter, the Archdevils shouldn’t be able to enter, nothing on the divine/cosmic scale should be able to pass through the Gate.  So yeah, they are a bit freaked out when an Archdevil finds a way through.

3) I’m portraying the Gods as uncaring.  They know they can simply make a new world, they are pretty freaking sure this one is headed towards destruction/corruption, so why keep pouring energy into it?  They may be Gods, but even Gods only have so much power.  Why not just cut their losses and start over?  Why not just take what they learned and make a better world next time?

4)  I think we just disagree here on a fundamental level.  No matter how much people preach, there will always be room for evil to find it’s way into the hearts of some people.  And the promise of power and importance is a tempting offer to your standard civilian or middle manager.

5) thank you

6) Oh I’m sure both sides would try to sneak all kinds of crap into the contract.  But I’d also think that the Archdevil might not put such a clause into the contract.  But maybe I’m missing how you put in a clause that allows you to take back what was given, that doesn’t void the contract. (Beyond the basic “upon death, your soul is forfeit and all lent power returns to its original owner” clause I’m sure all such contract would have)

1). So I think the issue, is you are viewing this as a one vs. all situation.  Like this one Archdevil is going to single handedly fight off an entire pantheon of Gods.  Instead view it as “if the Gods invade ANY of the 9 Hells/ directly attack ANY of the Archdevils, ALL OF the forces of hell would respond in fighting back as a unified force”.  Same could be said of if any of the Archdevils went to a God’s personal realm, ALL the Gods would respond by Marshaling their forces to defend the attacked God.  

The Gods may not always agree, but they do agree that a direct attack on one by the forces of evil, is a direct attack on them all.  The same idea is true of the Devils, they may fight and scheme against each other, but you can be sure they aren’t going to just let the Gods just show up and kill one of their own.

2) if you could get them all to agree to take it down, yeah they probably could (would still take a while).  But that also means that the anything and everything that was blocked from entering the plane, would now have access to it.  They would be opening the flood gates to all manner of terrors entering and ******* up the world.  And I refuse to make the Gods have limitless power, such that they could deal with the Archdevil issue while also fending off the cosmic horrors that would want to invade a prime material plane.

3) I fail to see how “good” is not represented.  The Hero literally sells his soul for the power and knowledge he needs to save the entire material plane.  The Gods bring stability and healing back to a world that is trying to recover from an apocalypse level event.  And of course the Gods are going to rewrite history, to cover their attempt to flee.  Would you follow a deity that had already proven willing to cut their losses and run when shit hits the fan?

4) I feel like you are viewing this through more of a “moral failings” lens and less of a “there are literal demons and devils who physically exist within this universe” lens.  That aside, of course there are going to be those who seek power from dark sources.  The world isn’t Eden, so what is your basic commoner supposed to do when the King’s 4th son kidnaps their daughter and is going to force her into marriage?  How would one react in a world where 80% of people have access to magic, and your average caster is equivalent to a level 8-10 wizard, while your skills are that of a level 1 or heck maybe you don’t have magic at all?

6) The cleric doesn’t instantly turn on the Archdevil.  But I do see your point.  I kind of figured that the cleric would eventually learn arcane magic on his own.  He needs the power from the Archdevil to deal with the problem in the here and now, but once he has time to breath and “relax” he would probably spend time learning the craft.  I do agree that the Archdevil would probably put something like that in, cleric might counter with limiting the scoop of that to not preventing him from ending the war.  Basically Archdevil (1) can’t team up with Archdevil (2), but cleric can’t act to stop Archdevil (1)’s plans that don’t directly relate to/ effect the war.

As far as the world building, I mean, there are a lot of real world theological works that exactly points to the adherence to religion/gods despite their apparent absence or lack of direct intervention into the material world, and rationalizing the importance of keeping to "good" at the core of a faith despite the "problem of evil" in the world.  Kotath is providing some (like literally) real constructive criticism.  I'll agree with Kotath though that for a game world it's "probably" fine unless you make it like a truly core issue at the heart of your game's story.  If that's the case, anyone with any background in comparative religions or well read in any particular faith's theology or moral reasoning will probably dice this up and blow the verisimilitude that's more important to a game than a clearly defined theology.  Doesn't mean you can't have a heroic lichlord stepping into the gap of left by what you're calling an absence.  Of course, it begs for a false idols rebuke, and there's plenty of mythology discussing how that goes.

I also don't get point six.  The Cleric loses their power, the Devil grants them Lichdom?  And from there the Lich deconstructs their arcane ability to retain the knowledge?  So it sounds like the Devil makes them basically a level 20 Sorcerers and in the Lich's spacetime, they MC somehow over into Wizard to develop parallel skills if the Devil takes them away.  I mean, I guess.  I just wonder if there was an "urgent here and now" to deal with whatever problem the Cleric needed the diabolical aid for, where's the free time to engage in Arcane work.  And is the Devil going to just make an investment and not keep track of what it's doing when it's idle?  

In mainstream game lore Devils are masters of transifiguring promoting/demoting their lessers.  It just seems this whole arrangement is like naked plot armor so your game can all praise "Cool Guy, Ultimate NPC."  Like, in the end, it's sort of "cool story, bro" but at what chapter do the PCs ever get to do anything of consequence.  World building is fine, but a lot of DMs make the mistake of setting up elaborate lore that's granted "factuality" from inception, and don't spend as much time giving the players any opportunity to do anything but witness the wonders of the DMs world.  It's really hard to get PC buy into worlds in which they're basically tourists.  Besides, myths are shifty things, things could come to pass where this story is actually not accurate or even altogether wrong.  Because all the lore is really stuff for the PCs to make use of as they adventure.  

So rather then get angry and try to “convince” you that this all makes sense.  I think it’s better to just admit that we view the concepts of “good and evil” differently.  That we view the strength of gods differently, along with a host of other differing opinions. 

I have no intention of making my Gods all powerful or Selflessly compassionate about mortals.  We are a source of power and amusement for them.  That’s all we ever have been, and all we ever will be.  There compassion and boons are simply tools to harvest higher quality souls.  

the whole campaign is built around the idea of “shades of gray”/ “thing always have another side”.  Like I have a villain who’s end goal would drive thousands of people off their ancestral land, and give that land to a menagerie of monsters.  True the monsters did own that land long before the ancestors of the people currently on it.  So who do you help?  They both have reasonable claim to the land, and they are not going to share it.  

Heck even with the Gods, it’s a question for them about how much time and effort are they willing/ able to put into “saving” this world.  Is it better to cut their losses use the stored power from the souls they already have to make a new world?  If they make a new world they can be sure the flaws of this one won’t effect the next interaction. Is it better to try and save this one?  What happens if they blow a huge chunk of their divine power saving this world, and the Archdevil has done too much damage?  If they use to much power and the world is doomed anyway, then they won’t have the strength to survive till a new world can be created and populated.  Can the Gods risk their own existence for the sake of mortals?

"Can the Gods risk their own existence for the sake of mortals?"  Yes.  Who else created the mortals in the first place?  They want to feed on high quality souls.  Why should they go somewhere else and create them?  Won't that take a lot of time and resources?   Whatever they do, and wherever they go, they are going to have to fight eventually.  I've never heard of someone who genuinely liked being homeless, except in D&D, where they get called "Murder-Hoboes".  Do you see the gods as being like that?  Not all gods are good, but there's something pretty close to all powerful out there, and whatever it might be, if its not the gods, who is it?  You talk about "shades of gray" but then you don't accept that there are any, and that no god could be selfless.

One of the bits of advice on writing is "don't be afraid to kill your darlings."

"Writing is a painful process and most experienced writers will tell you that good writing involves substantial rewriting. An essential part of the rewriting process is combing through your work and cutting out material that isn’t essential. Sometimes this means we have to lose things that we are proud of and attached to. When you edit out material like this, you are killing your darlings."

The super powerful Cleric/Lich is obviously your darling.

So it strikes me that we keep dancing around the core question.  Which is WHY?  Why do there HAVE to be all powerful beings?  Like the closest thing would be the primal forces, but even those could be “killed” if you met the absurd requirements.  Some of the Gods are simply personifications of the primal forces.  Like a God of death is not DEATH, it is how mortals perceive death.  Hence why you might find one god of death as being terrifying, and another as being warm and welcoming.

The Gods are unimaginably powerful compared to most mortals.  But they are not all powerful.  Gods have died before, some slain by other gods, some starved to death seeking worlds to be worshiped upon.  Some were simply forgotten for so long, they ceased to exist.  Some have even been slain by mortal hands.  

As for where mortals come from?  That depends on the plane you are on.  On many worlds the Gods brought forth life.  By creating it from nothing, by pulling existing life from a different plane, etc…. But there are some world that already had mortals on them when the Gods showed up.  They know not if the life evolved, or simply sprung into being one day.  But the mortals exist even in the Gods ignorance.

as for having to maybe cut out/ change the Cleric/Lich.  You may be right, but I want a better reason for removing such a character then “B…But the Gods HAVE to exist, and they HAVE to be all powerful.”

Ok.  Sure, but if the gods don't exist, then what does?  What exactly has the kind of power to kill those primal forces you mentioned, and how did they do it?  Evidence supports the conclusion that you see your Darling as having that kind of power eventually.

Let me paint you a story.  You are a farmer, you have made many a hen house in your life.  Some of those houses failed because of your poor Craftsmanship, some failed because the people who helped you make it started to argue over the hen house.  Heck some failed because forces outside of your control.  A few have even failed because the Chickens broke it from the inside out.  But every time a hen house fails, you learn and build a better hen house.  One that won’t fail like those before it.  So, what do you do when you find out a someone has found a flaw in your design and is using it to steal your hen house out from under you?  Perhaps you fight for it, perhaps in the fight the hen house ends up so damaged it becomes a lost cause.  Perhaps you need 40 keys to get into the hen house, of which you have 1 and your 39 friends all have one as well (a measure put into place to prevent any of you from ******* up this hen house… as has happened before).  Perhaps you can only convince 25 of your friends to help you, the other 15 think it a better use of your collective time to just make a new hen house.  I know that if it was me, I’d be damn sure that my next hen house didn’t have that same flaw to be exploited.

As to the threat.  The immediate threat is an Archdevil.  But if they open the gate (the thing barring their way onto the plane) they open up the passage for anything and everything that was previously barred from the plane to enter.

As to why evil has gained the advantage.  Simple “bad luck” and outside the box thinking.  The “good” guys fought a battle and it turned out worse then they could have ever imagined.  Around the same time someone on the Archdevil side of things figured out that their Lord possessed a unique ability that the rest of the Archdevils and the various Gods lacked.  They used this ability to slowly start pulling the Archdevil on to the plane (something that has never happened before and was thought impossible) through the divine gate.  With a vastly weakened “good” team, and the added power that comes from the Archdevil’s manifestation upon the world, the Archdevil’s forces begin to shift the tides of war further and further into their favor.  

I think what’s getting me.  Is that you seem to want me to either make the Gods so weak they need to be in fear of the Archdevils, or make them so strong they could destroy them without breaking a sweat.  You don’t seem to want a middle ground where both sides could win, hence why the eternal battle of Good vs. Evil has been raging for so long.

You want to kill TIME?  Lock it in a Demi plane where Time Stop is always in effect.  Or kill everything.  If there is nothing to mark the passage of time, then there is no way to prove time is passing/exists.  You want to kill DEATH?  Bless everything with eternal life.  If nothing ever dies, then death as a concept stops having meaning.

the problem with killing a primal force though.  Is that they don’t tend to stay killed.  As soon as something exists that time would have an effect on, time as a concept exists.  As soon as something dies, death as a concept comes back into being.  You can “Kill” a primal force only for so long as you can keep what “makes it” from happening.  

I fully agree!  So why can your Darling do it?

He can’t kill a Primal Force.  He can’t kill an Archdevil.  He wins against the Archdevil by hijacking the ritual to bring it into the plane, and trapping and splitting it’s soul among multiple Soul Jar/Imprisonment seals.  At no point does this guy directly fight a god level being.

Hell his plan to kill a god is just a modified version of the Archdevil summoning.  He’s banking that the Archdevil was the only being, who could survive having its “body” forcefully pulled through the Divine Gate.

Edit:  the Cleric turned Wizard wouldn’t be strong enough to beat an Archdevil.  But the Wizard turned Lich, who had centuries (if not millennia) to plan.  Yeah, that Lich could probably find a way to beat an Archdevil enough to get out of a contact with their soul intact.

So his plan requires the Archdevil to be stupid enough to let him mess with the his soul.  Archdevil don't have those.  They are Spirits, when you try to kill them, them they just reform back on their home plane.  You're going into their turf when you try this.  Your plan requires that there are no other entities of any kind to be foolish enough to not want to interfere.  I note that you made a mistake about the soul thing, and your plan requires that the Archdevil needs to make more of them than you do.  The player character is the one who has a soul, and Archdevils like to eat those.

This is something you do consistently.  Remember how many threads, in how many different forums, that you posted about playing as a Mastiff?  You wanted to be able to stay in Mastiff form and cast spells.  There wasn't a single person who said that would be ok.  There were a couple of times when I tried to stand up for the basic idea of an animal spell caster, but I was forced in the end to admit that paws aren't hands.  You refused to play a different animal with anything resembling hands.

Once you've got a cool idea, you can't seem to let it go, and once again, your Mastiff was a Darling, your Cleric/Lich is a Darling, and you refuse to see any of your Darlings die.

I disagree on the concept of a devil having a soul.  I’m partial to the idea that all beings have souls, those souls do not always match the physical bodies they inhabit, but they exist.  Hence why the Divine Gate works, it’s not based off the physical size of the thing trying to pass through it, it’s based off the size of the soul (how strong the being is).  That’s why you could totally bring a house cat the size of a mountain through the gate, but a God in the form of a pebble would never fit.

As to messing with the Archdevil soul.  Yes I very much believe that the Archdevil would fall for it.  The Devil’s whole plan revolves around the ritual, and the rituals effects on the Devil’s soul, to enter the material plane.

You mention some other powerful being messing with the ritual.  How did this being get through the gate?  How did it know the Archdevil’s plans?  How would it know where to look and how to foil the ritual?  Everyone of importance (Gods, devils, demons, other cosmic power level beings) “knows” that you couldn’t pass through the Gate.

There’s this subtle difference between this and the Mastiff posts.  One was crowd sourcing how to balance a homebrew race and item.  One is asking “hey which powerful evil being is best known for trading souls for arcane knowledge?”  

You also seem to not grasp that I’m fine with my “darlings” dying.  The cleric/Lich exists to ask some basic questions of the party.  “Do you trust gods who can, and have, left worlds they ruled over to die?”  “Who do you put more faith in, the God who’s been pushing their church to hide world history?  Or the person who literally sacrificed their soul to save the world?”.  It’s 1000% possible the party still choices to fight and kill this guy. (Cool they get to have an epic battle with a Lich) He’s a bitter old undead man who’s been holding a grudge against his former god for millennia.  It’s also 1000% possible that the party agrees with him and decides that maybe it’s time for a being who understands what it was like to be mortal, to finally hold a seat among the upper echelon of Gods.  There are countless versions of the story in between those two extreme, and I’m super excited to see how my players decide to handle these issues.  But I want the story to have depth.  Not have the big choices be between “save the orphans and puppies” vs. “support the puppy kicking, kitten drowning, rapist, murder hobo”

(as an aside.  The main issue with the Mastiff posts boiled down to most people arguing that if you wanted to play an awakened animal, you were stuck playing with nothing but the animals base stat block.  So no racial, no armor, no weapons, no casting, no items.  You are a dog, who is a dog, who is a dog.  You never became stronger because leveling up and learning new things was only allowed for the humanoid beings)

I would actually argue that confining is often better then killing.  Unless you can kill on the divine being's home plane of existence.

He defeats the Archdevil by being in the right place, at the right time, with the right knowledge, to deal with a ritual that was created by the Archdevil's cult and the Archdevil.  Like honestly its one of the most basic tropes, to have a character(s) just happen upon a problem and be able to deal with it, JUST before that problem would have become much worse.  How many of the official campaigns just HAPPEN to have the party bumble their way into stopping some horrible shit going down?  I know LMoP does, I'd bet my left nut that most of the modules are set up the same way "The party gets together to do A, but through a series of twists ends up eventually defeating the BBEG and accomplishing X"

Devils not having souls has a lot of deep lore precedent in D&D, as well as IRL "angels dancing on the head of pin" supernatural theology (also applies to Celestials).  "I feel 'different'" isn't really a thorough defense of what's been established in game and may be known by others players (who should be flexible to accept differences but may ask questions).  

You also don't seem to understand what "letting your darlings die" means in a worldbuilding/writing context.  It means you think this is a super cool idea that you're getting negative feedback on where there's a consensus saying the concept needs a serious rework or altogether abandonment and rather than take heed you double down.  In other words, you came here asking questions, you're overall concept got constructive critical objection and instead of growing your thoughts on the matter you make a hill to fight on and rather than really consider the feedback as advice, you choose to push back against it.

If you want to have a Big Bad Not Necessarily Evil Guy at the heart of your campaign, that's fine.  But really if you want to tie it somehow to 5e lore on Devils, you ought to do a deeper dive into the info offered you than you've exhibited.  If you want to use your own metaphysical "universe" or whatever, it's still "Cool story, bro, at what chapter do the PCs get to do anything."  It just seems there's a lot of emphasis setting up this deep mythology and there's no indication that the PCs will be interested in taking any of it on.  The PCs aren't going to need a "Big Picture" and where they go in their elementary adventures may radically reshape the big picture you thought you had.  Let the players do things in the world and build out from there (just like the DMG suggests), don't give them a packaged tour with predetermined destinations.  

I honestly don't know why you've asked the question since you don't seem to be accepting any feedback.  It'd be better to just write this up as lore for your world and exhibit it in the story and lore section without a question mark.