(From Planescape: Monstrous Compendium Appdendix III - 1998):

Haunting the deep reaches of the Ethereal and Astral Planes, the creatures known as devourers have a reputation for being among the most fearsome and loathsome of foes. For many planewalkers, they embody the true nature of evil (despite the fact that they don’t come from the Lower P1anes). See, Devourer have so little regard for anyone but themselves that they’re willing to steal the life essence of other beings to power their own magic.

Nothing about a devourer’s appearance gives the impression that it’s anything other than a horrible monster. A gaunt, skeletal creature of great height, it usually has a tiny figure trapped within what appears to be the bones of its rib cage. This captive is obviously powerless and clearly suffers great distress and pain, though he often has a look about him that indicates that he’s been imprisoned for a long, long time.

Combat: Most cutters fear the devourer, and with good reason. Sure, its large, crooked claws inflict 2d6 points of damage per strike, but even worse, its touch also drains one energy level. If wounded in battle, a devourer regains a number of hit points equal to those permanently lost by a victim from whom it drained a level.

What’s more, the creature can capture a berk’s entire life essence — truly a fate far worse than death. To steal a spirit, the devourer must roll a successful attack against its chosen victim. The sod must then make a successful saving throw vs. death magic to avoid the effect. If he fails the saving throw, he’s slain, and his spirit becomes trapped within the devourer. Onlookers can see it appear within the rib cage of the skeletal horror.

A captive spirit is completely unable to act and can’t be restored by a raise dead spell. Fact is, all it can do is serve as fuel for the devourer’s spells and abilities. The greater the level or Hit Dice of the trapped spirit, the more power it provides to its “host”. And because a devourer’s rib cage can hold only one spirit at a time, the creature always seeks to prey upon the mightiest victims it can find. While it holds a trapped spirit, a devourer can use the following spell-like powers at will, once per round: confusion, control undead, paralysis (as the wand), ray of enfeeblement, spectral hand (which it can use to drain levels, but not steal spirits), suggestion, summon shadow, and true seeing. Each level or Hit Die of the captive gives the devourer a total of five power uses. Once it has completely consumed the spirit in this manner, the sod’s life essence is destroyed forever and the devourer must find a new victim.

In addition to powering spell-like abilities, the captive spirit offers the devourer magical protection. If a cutter casts any of the following spells upon the creature (and if they penetrate its magic resistance), they affect the trapped spirit instcad: banishment, chaos, confusion, dispel evil, emotion, entrapment, ESP, fear, geas, holy word, imprisonment, magic jar, maze, quest, spirit wrack, trap the soul, and any type of charm, domination, hypnosis, or suggestion.

Note that some of these spells (including banishment, dispel evil, and entrapment) get rid or the trapped spirit, leaving the devourer’s rib cage empty. If this occurs, the creature can’t use any of its spell-like powers — or protect itself from the above-named spells — until it finds a new victim. And what becomes of the spirit, once released from its skeletal prison? Well, the basher’s still in the dead-book, but at least the spirit is free to go to the plane of its alignment (and can also be brought back to life with raise dead).

When encountered, a devourer will almost always have a captive spirit in its rib cage (85% change). The Dungeon Master should roll 3d4+3 to randomly determine how many Hit Dice or levels the victim has left for its captor to utilize.

Habitat/Society: Some folks believe that only one devourer exists, that it’s a unique creature. Wishful thinking — it’s very pretty well confirmed that that’s not the case. However, exactly what the creatures are is still a mystery. Are they a predator race found only on the Silver Void and the Misty Shore? Are they the magical creations of a viciously evil wizard? Are they manifestations of something else entirely — perhaps the most despicable thoughts and emotions that end up on the Astral? No one’s tumbled to the dark of it yet. Fact is, no one even knows if the things can communicate, though it’s been theorized that they’re telepathic.

When a planewalker stumbles across a devourer, the creature’s always alone (except for its captive spirit, of course). Nothing alive seems to tolerate the monsters’ presence; they even appear to abhor one another. Furthermore, devourers are encountered only when they’re out hunting for victims from whom they can drain levels or steal spirits. If the creatures do make lairs on the Astral and the Ethereal, such places’ve never been found. Perhaps that’s for the best.

Ecology: It’s not known how (or if) devourers reproduce. They subsist totally on life energy, needing no actual food or drink. Despite their appearance and their requirements for existence, they’re not undead.

Chant has it that a few bashers have encountered devourers on the Negative Energy Plane, but who’d believe the word of anyone who’d willingly go to such a horrible place? Others contend that while devourers spend most of their time on the Astral or the Ethereal, they do indeed make occasional trips to the Negative Energy Plane, for reasons unknown. Unlike undead, they have no direct connection to the plane (hence their ability to go to the Astral and even beyond, if they wished), but they do seem to make the odd “pilgrimage” now and then — no doubt because of their nature.

(From 3.5e Monster Manual I - 2003):

This tall, skeletal creature has strands of mummified flesh hanging from its bones. Imprisoned within the creature’s rib cage is a tiny figure, clearly in agony. 

Devourers are massive creatures every bit as evil as they look. They lurk on the Ethereal Plane and the Astral Plane, stalking both natives and travelers with equal sadistic glee. 

The tiny figure in the rib cage is the trapped essence of a slain opponent, which is consumed like firewood to sustain the monster’s unnatural life. A devourer is about 9 feet tall and weighs 500 pounds. 

Devourers speak Common. 

Combat: Even if it had no special abilities, a devourer would be a terrible opponent, for its bony claws can flay enemies alive. 

Energy Drain (Su): Living creatures hit by a devourer’s claw attack or spectral hand ability gain one negative level. The DC is 19 for the Fortitude save to remove a negative level. The save DC is Charismabased.

Trap Essence (Su): The devourer is named for its ability to consume an enemy’s life essence. To do so, it must forgo its normal melee attacks and make a trap essence attack. This requires a normal attack roll but deals no damage. The affected creature must succeed on a DC 19 Fortitude save or die instantly. The save DC is Charisma-based. 

A slain creature’s essence is trapped within the devourer’s ribs, and the tiny figure takes on that victim’s features. The trapped essence cannot be raised or resurrected, but a limited wish, miracle, or wish spell frees it, as does destroying the devourer. A devourer can hold only one essence at a time. 

The trapped essence provides a devourer with enough power to use five spell-like abilities for each Hit Die or level of the trapped creature. As this energy is expended, the twisted soul fades away until it evaporates completely. The trapped essence gains one negative level for every five times the devourer uses one of its spelllike abilities. When the essence’s number of negative levels equals the creature’s total Hit Dice or level, the essence is destroyed. If an essence is freed, the restored creature must succeed on a DC 19 Fortitude save for each negative level or lose that level permanently. 

Spell-Like Abilities: At the start of any encounter, the trapped essence within a devourer is assumed to have 3d4+3 levels (enough fuel for thirty to seventy-five uses). Once per round, a devourer can use one of the following abilities: confusion (DC 17), control undead (DC 20), ghoul touch (DC 15), lesser planar ally, ray of enfeeblement (DC 14), spectral hand, suggestion (DC 16), true seeing. Caster level 18th. The save DCs are Charisma-based. 

Spell Deflection (Su): The trapped essence provides a measure of magical protection. If any of the following spells are cast at the devourer and overcome its spell resistance, they affect the imprisoned essence instead: banishment, chaos hammer, confusion, crushing despair, detect thoughts, dispel evil, dominate person, fear, geas/quest, holy word, hypnosis, imprisonment, magic jar, maze, suggestion, trap the soul, or any form of charm or compulsion. In many cases, this deflection effectively neutralizes the spell (charming a trapped essence, for example, is useless). Some of these effects (banishment, for example) might eliminate the trapped essence, depriving the devourer of its spell-like abilities until it can consume another victim.

(From Volo's Guide to Monsters - 2016):

Of all the abominations Orcus has unleashed, devourers are among the most feared. These tall, mummy-like fiends wander the planes, consuming souls and, by example, spreading Orcus’s creed of replacing all life with everlasting death.

Instruments of Orcus. A lesser demon that proves itself to Orcus might be granted the privilege of becoming a devourer. The Prince of Undeath transforms such a demon into an 8-foot-tall, desiccated humanoid with a hollowed-out ribcage, then fills the new creature with a hunger for souls. Orcus grants each new devourer the essence of a less fortunate demon to power the devourer’s first foray into the planes. Most devourers remain in the Abyss, or on the Astral or Ethereal Plane, pursuing Orcus’s schemes and interests in those realms. When Orcus sends devourers to the Material Plane, he often sets them on a mission to create, control, and lead a plague of undead. Skeletons, zombies, ghouls and ghasts, and shadows are particularly attracted to the presence of a devourer.

Tormentors of Souls. Devourers hunt humanoids, with the intent of consuming them body and soul. After a devourer brings a target to the brink of death, it pulls the victim’s body in and traps the creature within its own ribcage. As the victim tries to stave off death (usually without success), the devourer tortures its soul with telepathic noise. When the victim expires, it undergoes a horrible transformation, springing forth from the devourer’s body to begin its new existence as an undead servitor of the monster that spawned it.

Fiendish Nature. A devourer doesn’t require air, food (other than souls), drink, or sleep.