Nabassu

From Mordenkainen's Tome of Foes (2018):

The insatiable nabassus prowl the multiverse in search of souls to devour. If they think they can kill a creature and consume its soul, they attack — even if that other creature is a demon, including another nabassu.

Hated Outcasts. Demons have few rules, and the murder of other demons hardly raises an eyebrow among these fiends. The act of devouring souls is something else. For this reason, most demons shun nabassus and force them to live on the fringes of the Abyss. There, nabassus pick off weaker demons or, if the situation warrants, gather in packs to take down larger prey. Some especially powerful nabassus even search for demon lords’ amulets.

Demonic Infiltrators. Whenever magic pulls demons from the Abyss to the Material Plane, nabassus try to get summoned so that they can embark on a feast of souls there. If a nabassu is summoned, it tries to break free so that it can devour the soul of its summoner and then set out to feed on the souls of whatever creatures it can catch. One way a summoner can avoid this fate is by providing a steady supply of souls to the nabassu, which can cause the demon to be cooperative — for as long as the supply lasts.

From Planescape: Monstrous Compendium Appendix I (1994):

Nabassu are creatures spawned in the Abyss but nurtured on other planes, where they grow and gain power by slaying and devouring humans. They communicate using telepathy.

Nabassu are a scourge of humanity. They are the only fiends that live a part of their lives on a foreign plane. Unlike other tanar’ri, nabassu seem to have no place in the Blood War. Once they return to the Abyss from their tour of carnage on the Prime Material, they will reside in one of the fortresses of the Abyss for the rest of their immortal existence.

What role these monsters play is unknown. Perhaps the slaying of humans and spilling of innocent blood somehow bring the tanar’ri power.

Combat: Nabassu go through two distinct phases, fledgling and mature, that have different combat abilities. However, in both phases the nabassu can attack with two claws (2d4 damage each) and bite (3d4 damage). They can attack with magic but prefer natural attacks, especially against a helpless enemy. These create greater terror in the victim and, therefore, greater pleasure for the nabassu.

Fledgeling: Fledglings begin life with AC 4 and 7 Hit Dice. When they enter another plane (usually the Outlands or the Prime), they gain 2 more hp. Each victim killed and eaten on the foreign plane adds 1 hp and a “half factor“ of AC. For example, a fledgling that devours two humans would have 7+4 HD and AC 3. After 18 meals it reaches full growth (AC -5 and 7+20 HD).

Immature nabassu do not gain the spell-like abilities available to other tanar’ri but have the following spell-like abilities: darkness, 15’ radius and its death gaze (once per day). The death gaze forces the target to save vs. spell or transform, over 10 days, into a ghast (or ghoul if the victim is a demihuman). The death of the nabassu or a remove curse spell reverses the process, but once the process is complete, the change is irrevocable.

Immature nabassu also have some thief abilities: move silently (40%), hide in shadows (50%), detect noise (55%), and backstab (×2). They are affected only by attacks from magic or cold-wrought iron weapons. Nabassu have Strength 19 (+7 damage adjustment).

Mature: At full growth, nabassu lose their fledgling abilities but gain those spell-like abilities available to all tanar’ri, plus the following spell-like powers: energy drain [by touch), regenerate (1 hp per hour), silence, 15‘ radius, and vampiric touch. Three times per day, a mature nabassu can attempt to gate in 2-20 manes, 2-5 cambions, or I mature nabassu with a 45% chance of success. When in the Abyss, mature nabassu can automatically summon 2-5 ghasts once per day. Mature nabassu can become ethereal at will, twice per day. Also, any living creature coming within 10’ must save vs. paralyzation or he paralyzed for 1dlo rounds (once per encounter). Mature nabassu have the same Strength and immunities as fledglings.