My overarching BBEG for my current campaign is Asmodeus. Now I know that no one (Even if multiple level 20s were there) can kill a god. So I thought the pcs could have some sort of adventures to find "divine shackles" that would chain asmodeus into eternal prison. It needs a lot of thought. But was is the best way to succsessfully fight against a god?
If you're the DM, and you want gods to be killable in your world, then they are killable. Decide how you want it to happen, and that's how it happens. It can be as easy or difficult as you like.
But if you want to go the shackles route. Maybe they could be more metaphoric shackles. Like the PCs need to find some ancient (homebrew) artifact or ritual which lets them bind him, or shuffle him off to some demi-plane where he can't influence the prime material for 10,000 years. There could be some interesting potential conflict with other gods, since if its powerful enough to do that to one god, any of them might be vulnerable. So even a god who opposes Asmodeus would be hesitant to see such power out in the world. Or go the Dr. Strange route, where the players need to find a way to trick him into making a deal where he'll stop doing whatever it is he's doing.
Take a look at the Divine Trammels and Rites of Prime Banishment from Critical Role. That should be nearly identical to what you're looking for.
Generally, fighting a God falls into a couple of categories:
1) Fight the Avatar of the God
In this, you're essentially defeating the God's representation in the Material Plane, which is just a very powerful monster. The victory is temporary, but good enough for an era of peace.
2) Get some help from an allied God
As with the "divine shackles", if the players have some kind of plot device, then they can exceed their mortal limitations temporarily. This could conceivably kill a god, but generally just results in banishment or enough damage to force the evil God in to near permanent exile.
3) Ascend to Godhood
The ultimate power move is to go full Kaiju battle and meet the deity on their own level. Maybe a tired god wants to retire and passes their domains on to the player(s), or perhaps the players discover a weapon that will forcibly take the divine power from the god. This scenario keeps the cosmic scales balanced by transferring domains from one entity to another.
One interpretation is that deities are the stewards of their eternal portfolio. The only way to "defeat" a deity permanently is to separate it from that portfolio and allow something else to absorb it, (usually another deity).
You can battle gods -- Tiamat is a god, and Asmodeus should be at a similar power level. They just generally don't stay dead.
That is true, but Tiamat is only a lesser god. And a lesser god with few mortal followers on the Prime Material plane, which means she is not as powerful as other gods such as Lathander or Selune who have millions across the continent. Plus she is a prisoner of Asmodeus in Avernus, not a guest. She is chained there with her power lessoned (though still insanely powerful.
Asmodeus however, is a greater god. One of the most powerful gods in fact. Sin is everywhere. Everyone who indulges just a little or does bad deeds gives a little power to Asmodeus. He canonically watches over multiple worlds (Toril, Oerth, etc.), and has the entire Hells to command and worship him. He consumed the divine spark of Azuth, greater god of magic, assimilating his power (though Azuth was reborn in the Second Sundering). Tieflings of Faerun were also claimed via a powerful ritual by the Toril Thirteen in the 1300's, making Asmodeus the patron deity of their race. His the ruby of his ruby rod staff is taken from the shard of evil, which I won't go into detail, but it's basically the purest malevolent force in the multiverse. Most mid-power gods would be at least CR 40, but Asmodeus is higher.
Umm, enough of the lore from me, sorry about that. Onto the question. It would be such a grand finale to fight Asmodeus himself, and I completely encourage trying it! It is possible, to fight him, but of course the PC's should be veeeery careful. If perhaps they were to gather an immortal ally or two to defeat him, that could certainly make all this possible.
How about this? The PC's beseech Moradin (or another great immortal craftsman), and request he create for them a pair of shackles or a prison capable of chaining a god.
The PC's gather allies. Perhaps they go to a demon lord for assistance in combat, or perhaps an archdevil who craves to usurp Asmodeus (basically all of them, but Mephistopheles or his daughter Glasya would be good candidates). Perhaps they could even get a god to fight at their side? It might take a god to permanently defeat a god. Maybe Tiamat herself wants to rebel against her jailor, or Torm has had enough of ol' Asmo tempting his paladins and decided to bring him down. Of course the problem with using wise lawful good or lawful neutral gods is that they might realise the fall of Asmodeus would likely mean the fall of the Nine Hells, freeing the demons to swarm across the planes. But if a god is bloodthirsty, vengeful or headstrong enough they might just go along with the PC's plot.
The characters find or receive artefacts to help them in the fight. E.g. The Book of Exalted Deeds (created by Bahamut, another potential ally?), a Holy Avenger sword, some other weapon that has bonuses against fiends, etc.
OR, an alternative. The fight is long and bloody. The party is on it's last legs, their mortal allies dead. A good half of the party is down, but behold! They wear down Asmodeus, driving the god to his knees, his devilish features bloody, full of shock and fear. They drive a magical sword through his chest, and he lies, gutted on the ground, struggling to maintain consciousness as his unholy blood seeps from his body. Then he smiles. A terrible smile with bloody lips, and he laughs, and he laughs, and he laughs.
The PC's try to cut him short, finishing him off, or bind him. But as his features darken, his body goes still, and his body disperses slowly in a cloud of embers, the laugh resounds throughout the space. Directionless. Deafening. Is it in their minds? Or is the very multiverse itself guffawing at them? It's hard to tell, until his voice speaks again.
"Did you really think it would be so easy?" He croons, before the party are spirited away, back to where they came from, or are left to stand where they stood. Alone. The space empty, motionless and silent. No one kills the king of devils. Evil is within us all. For as long as we remain, he remains.
The being they had fought was but an avatar. Another aspect of the great, terrible god that is Asmodeus. But then again, that could be a bit too much of a letdown? 😆
what i do is to tie down a chunk of the god's power on something else, for the sake of D&D gods have finite so you can have them do something big during or pre-fight, like planet making in terms of scale
or have something wound them in some way, maybe another god fought them before the party an lost leaving the BBEG wounds, or use a mcguffin on them
and also have the party track down some A-teir magic items for the fight (i'd say one each)
I most definetly am straying from common lore for Asmodeus. Like a lot. I said that Asmodeus previously known as Asteroth was a legendary adventurer, and was with a group called the Stewards of the Lost Age. (Drizzt Do Urden was apart of this group) Essentially he decended the "Voldemort" way and slowly went evil. Then by unknown means he ascended to a God.
That is my lore for him. It fits for my storyline for my campaign
OR, an alternative. The fight is long and bloody. The party is on it's last legs, their mortal allies dead. A good half of the party is down, but behold! They wear down Asmodeus, driving the god to his knees, his devilish features bloody, full of shock and fear. They drive a magical sword through his chest, and he lies, gutted on the ground, struggling to maintain consciousness as his unholy blood seeps from his body. Then he smiles. A terrible smile with bloody lips, and he laughs, and he laughs, and he laughs.
The PC's try to cut him short, finishing him off, or bind him. But as his features darken, his body goes still, and his body disperses slowly in a cloud of embers, the laugh resounds throughout the space. Directionless. Deafening. Is it in their minds? Or is the very multiverse itself guffawing at them? It's hard to tell, until his voice speaks again.
"Did you really think it would be so easy?" He croons, before the party are spirited away, back to where they came from, or are left to stand where they stood. Alone. The space empty, motionless and silent. No one kills the king of devils. Evil is within us all. For as long as we remain, he remains.
The being they had fought was but an avatar. Another aspect of the great, terrible god that is Asmodeus. But then again, that could be a bit too much of a letdown? 😆
Matt Mercer ran a very similar campaign idea to this in the final arc of Vox Machina. Vecna ascended and became a god, VM had to get aid from some of the other gods and then forge trammels, magical spears that could be driven into the form of Vecna. There was then a spell that they had to chant in order to bind Vecna behind the divine gate trapping him. Mechanically Vecna had to make a wisdom saving throw and then each trammel that was successfully driven into him increased the DC by 5. Weakening Vecna made driving the trammels in easier however the clever thing about this combat encounter was that if they had gone too far and "killed" Vecna his spirit would have escaped his physical form to be resurrected elsewhere in time. So it was a case of having to hurt him, but not too much. This part of the campaign itself was made up of the party hunting down the information, getting into the magical forge and then making the trammels before then fighting their way to Vecna.
You could do something similar but have the players have to scour the 9 hells for each item, maybe say that some thing when combined has the ability of binding a devil, created by a demon in the blood war but liberated by the lords of hell. None of the lords of Hell trusted another not to use it so it was split into 9 and the pieces hidden across the hells. The players can then negotiate, steal secretly or use force to get each piece of this item, some of the lords working with them wanting to see a rival chained, others trying to get the completed item for themselves in order to imprison a rival. You could even have a faction working to stop the party because they fear a Lord of Hell being trapped in this way will shift the blood war too far in one direction and risk Chaos winning. The Lords may well have scattered the piece they had to protect in other planes guarded by traps or monsters, maybe as the pieces are gathered together they start to become more detectable meaning the players have less and less time before they are tracked down.
Maybe the being that hired them is actually the demon that made it, recently reformed centuries after being destroyed by the devils that stole his weapon it wishes the party to complete its work, when this is fund out they have to decide, imprisoning a devil lord is good, but, it is for another evil being does this mean it is then evil. Is the enemy of my enemy my friend? Maybe Asmodeus offers the party a chance to instead change the device to imprison the demon lord that tricked them detailing out a contract and deal for the players to enter.
One thing would be to take a look at the old Gauntlet Legends/Dark Legacy videogame, at least in terms of helping the party have a chance at success. Each of the bosses in that game had a weakness that was found in one of the other realms, and using it on them would immediately drop their health a good bit as well as removing or reducing some of their attacks. Transposing this on Asmodeus could be a Cleric ritual to 'break' the Ruby Rod, which would diminish his power and make him defeatable, or in a more grimdark direction, the ritual could involve the willing sacrifice of a Tiefling of Asmodeus' line to bind his essence into the corpse, which gives you a moral choice on fighting him.
I'm reminded of the end of TES IV: Oblivion. The last boss is also an evil demon God that you can't kill, but rather then try to subdue him, the game merely puts Mehrunes Dagon as an unkillable, super-powerful obstacle trying to keep the player from a secondary goal (get to the Temple of the One with Martin).
So maybe instead of trying to kill Asmodeus, or shackle him (which amounts to much the same thing in this context), maybe give the players an objective and put Asmodeus between them and it. Maybe they have to figure out how to do something like a puzzle or a riddle while fighting for their lives, OR maybe they're just trying to distract Asmodeus from something else going on for long enough for some NPC to complete a powerful ritual or something, so the mission is the players need to see how long they can survive against Asmo. Those are the top two ways I would run high level combat against a godlike enemy. The challenge mostly boils down to survival, not necessarily victory.
I don't think Asomodeus is actually a god. He's a devil so if he's killed outside of the nine hell's he is simply banished there and he is immensely powerful but he does not have the same immortality a god does. Power wise he likely matches or exceeds many gods but he is not a god he is something else.
As for killing gods, you can do it but they tend to return when their portfolio gains power or becomes more relevant (like auril returning next winter) . The only way gods have permanently died is by another god taking their position or being killed by another god but even in that case they can be brought back by defeating the god who took their folio, being resurrected by a more powerful god or a cataclysmic magical event like the sundering. Finally similarly to devils if they are killed outside of their realms such as in the form of an avatar they are merely banished to their realms.
That is how it has tended to work in DND. Personally I think your players, if they could get to the bottom of the hells or use high magic to bring his true form some where else they could kill asomodeus if they are capable of reducing him to 0 hitpoints. The issue is that I think he would be promptly resurrected by primus or another lawful god seeking to fend off the demons and avoid a war of succession among the devil.s
Between 4th edition and 5th edition he did become a legitimate god of the Faerûnian pantheon. Initially he fed off of the divine spark of Azuth, the god of magic, whom he imprisoned, to parasitically empower himself to become a greater god. Later, however, he made a deal to free Azuth in exchange for devouring the divine spark of a long dead god of Unther, and in doing so, became a legitimate deity in his own right. This all occured in the Brimstone Angels novels, and in all 5e modules and lorebooks he is also described as a god. God of Indulgence, patron deity of tieflings and ruler of the Nine Hells. But yes, he is simultaneously also an archdevil.
I would think to make this a series of adventures. If you want to shackle him, you first need shackles that are either physically strong enough on their own to hold him, or shackles so well made they can hold the enchantments needed to bind him, This could be a quest in itself, possibly searching out the finest dwarven smiths or beseeching the gods for them. Then you need the enchantments themselves. Do these come from clerics of gods such as Torm, or maybe followers of Tiamat who want to free her from her prison? Then how do you get to the Nine Hells, and how do you go unobserved while there? Finally, how do you engage Asmodeus in a fight in such a way that you can defeat him or slip the shackles on him?
All of these things could be part of an ongoing campaign. Sounds pretty epic.
Between 4th edition and 5th edition he did become a legitimate god of the Faerûnian pantheon. Initially he fed off of the divine spark of Azuth, the god of magic, whom he imprisoned, to parasitically empower himself to become a greater god. Later, however, he made a deal to free Azuth in exchange for devouring the divine spark of a long dead god of Unther, and in doing so, became a legitimate deity in his own right. This all occured in the Brimstone Angels novels, and in all 5e modules and lorebooks he is also described as a god. God of Indulgence, patron deity of tieflings and ruler of the Nine Hells. But yes, he is simultaneously also an archdevil.
Well then you might also be able to demote him back to arch devil from god if you defeat him and bring the demoted god Nanna-Sin to claim back his divinity or you'd create could create a new god.
Between 4th edition and 5th edition he did become a legitimate god of the Faerûnian pantheon. Initially he fed off of the divine spark of Azuth, the god of magic, whom he imprisoned, to parasitically empower himself to become a greater god. Later, however, he made a deal to free Azuth in exchange for devouring the divine spark of a long dead god of Unther, and in doing so, became a legitimate deity in his own right. This all occured in the Brimstone Angels novels, and in all 5e modules and lorebooks he is also described as a god. God of Indulgence, patron deity of tieflings and ruler of the Nine Hells. But yes, he is simultaneously also an archdevil.
Well then you might also be able to demote him back to arch devil from god if you defeat him and bring the demoted god Nanna-Sin to claim back his divinity or you'd create could create a new god.
Sounds like a good idea to me!
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Hello fellow DMs! Quick question.
My overarching BBEG for my current campaign is Asmodeus. Now I know that no one (Even if multiple level 20s were there) can kill a god. So I thought the pcs could have some sort of adventures to find "divine shackles" that would chain asmodeus into eternal prison. It needs a lot of thought. But was is the best way to succsessfully fight against a god?
A New DM up against the World
You can battle gods -- Tiamat is a god, and Asmodeus should be at a similar power level. They just generally don't stay dead.
If you're the DM, and you want gods to be killable in your world, then they are killable. Decide how you want it to happen, and that's how it happens. It can be as easy or difficult as you like.
But if you want to go the shackles route. Maybe they could be more metaphoric shackles. Like the PCs need to find some ancient (homebrew) artifact or ritual which lets them bind him, or shuffle him off to some demi-plane where he can't influence the prime material for 10,000 years. There could be some interesting potential conflict with other gods, since if its powerful enough to do that to one god, any of them might be vulnerable. So even a god who opposes Asmodeus would be hesitant to see such power out in the world. Or go the Dr. Strange route, where the players need to find a way to trick him into making a deal where he'll stop doing whatever it is he's doing.
Take a look at the Divine Trammels and Rites of Prime Banishment from Critical Role. That should be nearly identical to what you're looking for.
Generally, fighting a God falls into a couple of categories:
1) Fight the Avatar of the God
In this, you're essentially defeating the God's representation in the Material Plane, which is just a very powerful monster. The victory is temporary, but good enough for an era of peace.
2) Get some help from an allied God
As with the "divine shackles", if the players have some kind of plot device, then they can exceed their mortal limitations temporarily. This could conceivably kill a god, but generally just results in banishment or enough damage to force the evil God in to near permanent exile.
3) Ascend to Godhood
The ultimate power move is to go full Kaiju battle and meet the deity on their own level. Maybe a tired god wants to retire and passes their domains on to the player(s), or perhaps the players discover a weapon that will forcibly take the divine power from the god. This scenario keeps the cosmic scales balanced by transferring domains from one entity to another.
One interpretation is that deities are the stewards of their eternal portfolio. The only way to "defeat" a deity permanently is to separate it from that portfolio and allow something else to absorb it, (usually another deity).
That is true, but Tiamat is only a lesser god. And a lesser god with few mortal followers on the Prime Material plane, which means she is not as powerful as other gods such as Lathander or Selune who have millions across the continent. Plus she is a prisoner of Asmodeus in Avernus, not a guest. She is chained there with her power lessoned (though still insanely powerful.
Asmodeus however, is a greater god. One of the most powerful gods in fact. Sin is everywhere. Everyone who indulges just a little or does bad deeds gives a little power to Asmodeus. He canonically watches over multiple worlds (Toril, Oerth, etc.), and has the entire Hells to command and worship him. He consumed the divine spark of Azuth, greater god of magic, assimilating his power (though Azuth was reborn in the Second Sundering). Tieflings of Faerun were also claimed via a powerful ritual by the Toril Thirteen in the 1300's, making Asmodeus the patron deity of their race. His the ruby of his ruby rod staff is taken from the shard of evil, which I won't go into detail, but it's basically the purest malevolent force in the multiverse. Most mid-power gods would be at least CR 40, but Asmodeus is higher.
Umm, enough of the lore from me, sorry about that. Onto the question. It would be such a grand finale to fight Asmodeus himself, and I completely encourage trying it! It is possible, to fight him, but of course the PC's should be veeeery careful. If perhaps they were to gather an immortal ally or two to defeat him, that could certainly make all this possible.
Maybe Tiamat herself wants to rebel against her jailor, or Torm has had enough of ol' Asmo tempting his paladins and decided to bring him down. Of course the problem with using wise lawful good or lawful neutral gods is that they might realise the fall of Asmodeus would likely mean the fall of the Nine Hells, freeing the demons to swarm across the planes. But if a god is bloodthirsty, vengeful or headstrong enough they might just go along with the PC's plot.
Anyway, hope that is helpeful! :)
OR, an alternative. The fight is long and bloody. The party is on it's last legs, their mortal allies dead. A good half of the party is down, but behold! They wear down Asmodeus, driving the god to his knees, his devilish features bloody, full of shock and fear. They drive a magical sword through his chest, and he lies, gutted on the ground, struggling to maintain consciousness as his unholy blood seeps from his body. Then he smiles. A terrible smile with bloody lips, and he laughs, and he laughs, and he laughs.
The PC's try to cut him short, finishing him off, or bind him. But as his features darken, his body goes still, and his body disperses slowly in a cloud of embers, the laugh resounds throughout the space. Directionless. Deafening. Is it in their minds? Or is the very multiverse itself guffawing at them? It's hard to tell, until his voice speaks again.
"Did you really think it would be so easy?" He croons, before the party are spirited away, back to where they came from, or are left to stand where they stood. Alone. The space empty, motionless and silent. No one kills the king of devils. Evil is within us all. For as long as we remain, he remains.
The being they had fought was but an avatar. Another aspect of the great, terrible god that is Asmodeus. But then again, that could be a bit too much of a letdown? 😆
what i do is to tie down a chunk of the god's power on something else, for the sake of D&D gods have finite so you can have them do something big during or pre-fight, like planet making in terms of scale
or have something wound them in some way, maybe another god fought them before the party an lost leaving the BBEG wounds, or use a mcguffin on them
and also have the party track down some A-teir magic items for the fight (i'd say one each)
I most definetly am straying from common lore for Asmodeus. Like a lot. I said that Asmodeus previously known as Asteroth was a legendary adventurer, and was with a group called the Stewards of the Lost Age. (Drizzt Do Urden was apart of this group) Essentially he decended the "Voldemort" way and slowly went evil. Then by unknown means he ascended to a God.
That is my lore for him. It fits for my storyline for my campaign
Edit: I made him the god of Trickery
A New DM up against the World
That would be harsh lol
A New DM up against the World
I suppose another question that poses for me then, is how would the party gain the favor of a god (Bahamut, Mephistopheles, Torm, Tiamat, etc.)
A New DM up against the World
Matt Mercer ran a very similar campaign idea to this in the final arc of Vox Machina. Vecna ascended and became a god, VM had to get aid from some of the other gods and then forge trammels, magical spears that could be driven into the form of Vecna. There was then a spell that they had to chant in order to bind Vecna behind the divine gate trapping him. Mechanically Vecna had to make a wisdom saving throw and then each trammel that was successfully driven into him increased the DC by 5. Weakening Vecna made driving the trammels in easier however the clever thing about this combat encounter was that if they had gone too far and "killed" Vecna his spirit would have escaped his physical form to be resurrected elsewhere in time. So it was a case of having to hurt him, but not too much. This part of the campaign itself was made up of the party hunting down the information, getting into the magical forge and then making the trammels before then fighting their way to Vecna.
You could do something similar but have the players have to scour the 9 hells for each item, maybe say that some thing when combined has the ability of binding a devil, created by a demon in the blood war but liberated by the lords of hell. None of the lords of Hell trusted another not to use it so it was split into 9 and the pieces hidden across the hells. The players can then negotiate, steal secretly or use force to get each piece of this item, some of the lords working with them wanting to see a rival chained, others trying to get the completed item for themselves in order to imprison a rival. You could even have a faction working to stop the party because they fear a Lord of Hell being trapped in this way will shift the blood war too far in one direction and risk Chaos winning. The Lords may well have scattered the piece they had to protect in other planes guarded by traps or monsters, maybe as the pieces are gathered together they start to become more detectable meaning the players have less and less time before they are tracked down.
Maybe the being that hired them is actually the demon that made it, recently reformed centuries after being destroyed by the devils that stole his weapon it wishes the party to complete its work, when this is fund out they have to decide, imprisoning a devil lord is good, but, it is for another evil being does this mean it is then evil. Is the enemy of my enemy my friend? Maybe Asmodeus offers the party a chance to instead change the device to imprison the demon lord that tricked them detailing out a contract and deal for the players to enter.
Takhisis of Dragon Lance is a greater god and Raistlin would have slain her had his brother not convinced him that it wouldn't be a good idea.
One thing would be to take a look at the old Gauntlet Legends/Dark Legacy videogame, at least in terms of helping the party have a chance at success. Each of the bosses in that game had a weakness that was found in one of the other realms, and using it on them would immediately drop their health a good bit as well as removing or reducing some of their attacks. Transposing this on Asmodeus could be a Cleric ritual to 'break' the Ruby Rod, which would diminish his power and make him defeatable, or in a more grimdark direction, the ritual could involve the willing sacrifice of a Tiefling of Asmodeus' line to bind his essence into the corpse, which gives you a moral choice on fighting him.
However, it could make for an EPIC One-shot.
I'm reminded of the end of TES IV: Oblivion. The last boss is also an evil demon God that you can't kill, but rather then try to subdue him, the game merely puts Mehrunes Dagon as an unkillable, super-powerful obstacle trying to keep the player from a secondary goal (get to the Temple of the One with Martin).
So maybe instead of trying to kill Asmodeus, or shackle him (which amounts to much the same thing in this context), maybe give the players an objective and put Asmodeus between them and it. Maybe they have to figure out how to do something like a puzzle or a riddle while fighting for their lives, OR maybe they're just trying to distract Asmodeus from something else going on for long enough for some NPC to complete a powerful ritual or something, so the mission is the players need to see how long they can survive against Asmo. Those are the top two ways I would run high level combat against a godlike enemy. The challenge mostly boils down to survival, not necessarily victory.
I don't think Asomodeus is actually a god. He's a devil so if he's killed outside of the nine hell's he is simply banished there and he is immensely powerful but he does not have the same immortality a god does. Power wise he likely matches or exceeds many gods but he is not a god he is something else.
As for killing gods, you can do it but they tend to return when their portfolio gains power or becomes more relevant (like auril returning next winter) . The only way gods have permanently died is by another god taking their position or being killed by another god but even in that case they can be brought back by defeating the god who took their folio, being resurrected by a more powerful god or a cataclysmic magical event like the sundering. Finally similarly to devils if they are killed outside of their realms such as in the form of an avatar they are merely banished to their realms.
That is how it has tended to work in DND. Personally I think your players, if they could get to the bottom of the hells or use high magic to bring his true form some where else they could kill asomodeus if they are capable of reducing him to 0 hitpoints. The issue is that I think he would be promptly resurrected by primus or another lawful god seeking to fend off the demons and avoid a war of succession among the devil.s
Between 4th edition and 5th edition he did become a legitimate god of the Faerûnian pantheon. Initially he fed off of the divine spark of Azuth, the god of magic, whom he imprisoned, to parasitically empower himself to become a greater god. Later, however, he made a deal to free Azuth in exchange for devouring the divine spark of a long dead god of Unther, and in doing so, became a legitimate deity in his own right. This all occured in the Brimstone Angels novels, and in all 5e modules and lorebooks he is also described as a god. God of Indulgence, patron deity of tieflings and ruler of the Nine Hells. But yes, he is simultaneously also an archdevil.
I would think to make this a series of adventures. If you want to shackle him, you first need shackles that are either physically strong enough on their own to hold him, or shackles so well made they can hold the enchantments needed to bind him, This could be a quest in itself, possibly searching out the finest dwarven smiths or beseeching the gods for them. Then you need the enchantments themselves. Do these come from clerics of gods such as Torm, or maybe followers of Tiamat who want to free her from her prison? Then how do you get to the Nine Hells, and how do you go unobserved while there? Finally, how do you engage Asmodeus in a fight in such a way that you can defeat him or slip the shackles on him?
All of these things could be part of an ongoing campaign. Sounds pretty epic.
Well then you might also be able to demote him back to arch devil from god if you defeat him and bring the demoted god Nanna-Sin to claim back his divinity or you'd create could create a new god.
Sounds like a good idea to me!