The native plane of a larva or lemure

dnd-5emonstersplanessoul

When a creature dies, and it has a soul, their soul departs to the plane of their deity or their alignment. In case of evil creatures, they may end up in the lower planes.

For Hades, the DMG (p. 63) says:

[…] Hades is the destination of many souls that are unclaimed by the gods of the Upper Planes or the fiendish rulers of the Lower Planes. These souls become larvae and spend eternity in this place […]

The larva is given a stat block of a Medium fiend, neutral evil.

For the lemure, the MM (p. 67) states under "The Infernal Hierarchy":

Lemures. The lowest form of devil, lemures are the twisted and tormented souls of evil and corrupted mortals. A lemure killed in the Nine Hells is only permanently destroyed if it is killed with a blessed weapon or if its shapeless corpse is splashed with holy water before it can return to life.

Promotion and Demotion. When the soul of an evil mortal sinks into the Nine Hells, it takes on the physical form of a wretched lemure. […]

The lemure is likewise given a stat block, as a Medium fiend (devil), lawful evil.

What is the native plane of the larva or lemure that is a transformed form of a soul native to another plane?

I think the interpretation has concrete conseqeunces:

  • If the larva or lemure is the soul and on its native plane, and
    destroying it would destroy the soul; no re-animation, not even
    with Wish, could bring the original creature back to life.
  • If, however, the native plane of the soul is the plane of the
    original creature, it should depart to that plane upon destruction of
    the physical form of the larva or lemure, and should be available to
    be resurrected then.

Best Answer

In the default cosmology lemures are native to the Nine Hells and Larvae are native to Hades

Lemures

Lemures are created on the Nine Hells, either from souls of people who have made bargins with Devils, those claimed by the Hells due to their evil alignment, or those explicity killed by the Narzugon.

MToF on lemures in the Nine Hells:

Devils constantly strive to recruit mortals into their ranks by offering them rewards in return for their service. [...] When a cultist dies, its soul emerges in the Nine Hells and becomes another of the Blood War’s immortal soldiers. Most of the evil souls consigned to an afterlife in the Nine Hells become lemures, which make up the vast majority of the hellish forces, but some mortal recruits who willingly accept a contract offer from a powerful devil can arrive as a lesser devil.

Once they are a lemure, they are now a devil, a devil native to the Nine Hells (ie if they are killed outside of the Nine Hells they follow the normal rules for being reconstituted on the Nine Hells). The soul is now native to the Nine Hells.

MToF entry on Narzugon:

If this damage kills a creature, the creature's soul rises from the River Styx as a lemure in Avernus in ld4 hours. If the creature isn't revived before then, only a wish spell or killing the lemure and casting true resurrection on the creature's original body can restore it to life. Constructs and devils are immune to this effect.

It should be noted, that the exception here for wish is specific to lemures transformed by the Narzugon, and does not apply to all lemures.

Larvae

Larvae are created by transporting a soul to Hades, or by a creature visiting Hades and being transformed by the plane itself.

The Dungeon Masters Guide section on Hades specifies:

[...] Hades is the destination of many souls that are unclaimed by the gods of the Upper Planes or the fiendish rulers of the Lower Planes. These souls become larvae and spend eternity in this place that lacks a sun, a moon, stars, or seasons.

Creatures that die on Hades are also transformed:

Optional Rule: Vile Transformation

[...] If the creature reaches six levels of exhaustion, it doesn't die. Instead, the creature is permanently transforms into a larva, whereupon all levels of exhaustion afflicting the creature are removed. [...]

Hades is crawling with larvae. Night hags, liches, and rakshasas harvest them for use in vile rituals. Other fiends like to feed on them.

The creature the soul is in is now a fiend, and native to the plane of Hades, thus the soul is now native to the plane of Hades.

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