Atahsaia

Forever Starved. The átahsaia cannot die of hunger or thirst, no matter how long it has gone without food and drink. If the átahsaia eats another fiend or humanoid, it consumes the creature whole leaving behind no traces of it, besides any magical equipment on the creature's body.

Golden-Tongued. The átahsaia has advantage on all Deception checks against any non-fiend creature.  

Innate Spellcasting. The átahsaia's innate spellcasting ability is Charisma (spell save DC 16). It can innately cast the following spells, requiring no material components:

At will: minor illusion, primal savagery

2/day: hunter's mark,

Voracious Focus. Whenever attacking a fiend or humanoid, the átahsaia gains a bonus to its attack roll equal to the amount of days it has gone without eating up until a maximum bonus of +3.  

Multiattack. The átahsaia makes three attacks with its knife or three with its bite. 

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage. 

Knife. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (3d4 + 4) piercing damage. 

Consume. The átahsaia eats the corpse of a large or smaller creature which has been dead for no longer than one week. Upon doing so, it rolls a number of d10 equal to the consumed creature's level or CR and gains that amount of hit points up until a maximum of 6d10. If the consumed creature's level or CR is 1 or below, or if the creature is not a fiend or humanoid, the átahsaia instead gains no hit points.  

The átahsaia can also attempt to swallow a creature that's alive. To do so, the átahsaia makes a Strength check against a creature it has grappled of medium size or smaller. If the átahsaia has a higher score than the targeted creature, the creature then becomes swallowed. While swallowed, a creature is blinded and restrained, obtains three-quarters cover from any attacks and other effects outside the átahsaia, and accrues one point of exhaustion for each turn they spend swallowed. If a creature is killed by exhaustion in this manner, the átahsaia gains hit points as if consuming a corpse, unless the creature is neither a fiend or a humanoid. Only one creature can be swallowed in this way at any given time. If the átahsaia takes 20 points of damage from a single source, or if the átahsaia takes any points of poison damage, it must make a DC 17 Constitution saving throw to keep the consumed creature swallowed. On a failed save, the creature is spat up and falls prone within 5 feet of the átahsaia. If the átahsaia dies, a swallowed creature is no longer restrained by it, and can escape from the corpse using 5 feet of movement, exiting prone.

Human Transformation. The átahsaia can magically transform its appearance into any humanoid it has previously seen adopting all known mannerisms, provided it spent 5 minutes observing it, or 1 minute directly speaking to or interacting with it. If the átahsaia takes any damage inflicted by a weapon or spell attack from another creature, it must make a DC 15 Constitution saving throw in order to stay disguised. On a failed save, the átahsaia transforms back into its regular appearance.      

A creature attempting to discern the falseness of the átahsaia's disguise must succeed on a DC 18 Insight check. 

Aura of Famine. As a reaction to whenever another creature enters within 10 feet of the átahsaia, the átahsaia can exert its gluttonous presence upon them. The targeted creature must make a DC 16 Constitution saving throw or be consumed with hunger until the end of their next turn. While affected in this way, the creature's speed is halved, and they cannot take reactions.