How can you keep a devil from reforming in hell?

Question

The lore in the monster entry for devils says:

If it dies outside the Nine Hells, a devil disappears in a cloud of sulfurous smoke or dissolves into a pool of ichor, instantly returning to its home layer, where it reforms at full strength.

Is there a way a player character can prevent this from happening?

The following lore from Monsters of the Universe Drow Shadowblade entry explains how to do it for lesser demons, even if that may not be available to PCs:

A shadowblade harnesses a dark magic that is said to arise from a fiendish ritual in which the drow kills a lesser demon and mystically prevents it from reforming in the Abyss. This ritual creates a shadow demon and infuses the drow with shadow magic.

Is there similar lore for how to do this with devils?

Answer 1

Kill it in Hell.

As you mention in the question, if the devil dies while outside Hell, it just reforms right back. However, if it dies in Hell, it dies for real. So just kill it in Hell. Simple, right? Sort of. The idea here is to inflict some sort of delayed damage, giving you time to banish the devil back to Hell with a spell like banishment or plane shift. I’m sure someone more creative could come up with some reliable methods. The simplest solution I can think of is a damage over time spell such as acid arrow:

Make a ranged spell attack against the target. On a hit, the target takes 4d4 acid damage immediately and 2d4 acid damage at the end of its next turn.

Get the devil close to death, hit them with an acid arrow, hope you don’t kill it with that hit, then banish it and hope the next turn damage from acid arrow kills it. Other methods might look something like using hold monster and sticking dynamite in its mouth, then banishing.

Answer 2

Kill it in Barovia

Specifically in Barovia, souls cannot leave the place upon their body's death:

In Barovia, the souls of the dead are as trapped as the souls of the living. They become caught in the mists and can’t travel to the afterlife. (CoS 24)

Even the most powerful magic cannot change this:

No spell—not even wish—allows one to escape from Strahd's domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence. These restrictions apply to magic items and artifacts that have properties that transport or banish creatures to other planes. (ibid)

Thus, killing a devil in Barovia should be able to successfully separate the devil's soul from its body while not permitting the soul to return to Hell to reform a new body. However, that in itself does not help with the process of turning it into a shadow devil or whatever the diabolic equivalent is of a shadow demon.

See also: What happens if Majesto (an imp in Curse of Strahd) dies?

What happens to a devil that dies in an area warded against planar travel?

True Polymorph then Magic Jar

If you have access to True Polymorph, first change the devil into a humanoid - a Commoner with low saves will be ideal. Once it is a humanoid, there are a number of options for separating its body from its soul, but most of these involve killing it or reducing it to 0hp, which would end the polymorph spell. One thing you could do is restrain it, then cast magic jar and attempt to possess it from the jar, so that you are now in its body and its soul is trapped in the jar. Hopefully your ritual works with a separated soul and the body itself doesn't need to be dead.

As before, this presumes that you know how to do the requisite ritual. Also, this relies on the language in magic jar that says "The only action you can take is to project your soul up to 100 feet out of the container...attempting to possess a humanoid's body." The target body is, at the moment, a humanoid thanks to the polymorph spell. Whether it is actually a 'humanoid's body', and what the meaning of that possessive is, is something for your DM to interpret. I can imagine some would say 'It is not a humanoid's body, it is a devil's body that is currently polymorphed into a humanoid.'

Answer 3

There's no lore for it

I could not find any lore that would allow one to keep a devil from returning to the hells upon death.

However, there are three spells that could be tried to achieve this. Magic circle is pretty solid as to rules as written, and the fiend trapped in the circle is also a well-established trope. The other two are mostly depending on DM buy-in. With that being the case, if you want this in the campaign, it probably would be best to negotiate a way with the DM of how it could be made to work with a special quest/ritual/item/spell, or, if you are the DM, come up with such.

The alternative obviously is that there is no way to stop a devil from reforming in hell.

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Here are the details on the possible options with spells:

1. Inverted Magic Circle

Magic Circle says:

The creature can’t willingly enter the cylinder by nonmagical means. If the creature tries to use teleportation or interplanar travel to do so, it must first succeed on a Charisma saving throw. [...] When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder

The devil tries to leave the circle by interplanar travel, so it will get a Charisma saving throw. If it fails, the circle holds it back.

It is not spelled out if turning to smoke and ichor and instantly returning to its home layer are all one thing, or are a sequence of effects, so the DM must decide this. If it is all one thing, then the circle preventing planar travel should prevent the whole process. If it are separate steps, the devil would dissolve into smoke or ichor, but could not return, and would remain as a cloud of smoke or puddle of ichor in the circle, for as long as the circle remains active. Either way, it would prevent escape.

2. Disintegrate

Disintegrate says:

The target is disintegrated if this damage leaves it with 0 hit points.

A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust.

If you condsider being turned to dust as a transformation rather than dying (like when you get turned to stone), then depending on the timing of effects you could stop the devil from returning to hell, because it never dies. If you think being turned to dust is dying in dictionary sense of "stopping to live", the dust can turn to smoke or ichor and the devil will return to hell. Which of the two it is will be up to the DM, as the rules for dying never define what it exactly means.

3. Wish

Of course, no "can you" answer is complete without mentioning off-label use of wish with the DM approving such use. A cleric's divine intervention combined with DM approval might work, too.