Unnamable Horror

Immutable Form. The unnamable horror is immune to any spell or effect that would alter its form.

Ultimate Shapechanger. When the unnamable horror drops a creature to 0 hit points, it can use its action to absorb that creature’s essence. The creature’s body is destroyed and the unnamable horror can polymorph into that creature on its next turn. The unnamable horror gains the statistics of the absorbed creature, including its AC, speed, Strength, Dexterity, Constitution, and Intelligence scores unless its natural scores are higher, as well as gaining new skills, proficiencies, and languages. It retains all its traits and actions and gains all of the absorbed creature’s traits and actions. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Will to Survive. When the unnamable horror drops to half its hit points or fewer, it can split its body in two in an effort to escape. As a bonus action, the part of its body that isn’t in immediate danger can detach, halving its size, grow legs or some other appendage of locomotion and move up to half its speed without provoking attacks of opportunity.

Regeneration. The unnamable horror regains 10 hit points at the beginning of each of its turns. If the unnamable horror takes fire damage, this trait doesn't function at the start of the horror's next turn. The horror dies only if it starts its turn with 0 hit points and doesn't regenerate.

Multiattack. The unnamable horror makes three attacks: one with its bite and two with its whips. Alternatively, it can make two whip attacks and use its acid spray.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) piercing damage. If the target is a creature, it must succeed on a DC 14 Constitution saving throw or be infected with a disease - the unnamable horror infection.

The infection takes 2d10 hours to complete. During this time the host is clueless to the disease. When it comes to fruition, the host dies and becomes a new unnamable horror in the form of the host.

The infection can be stopped before fruition using greater restoration or similar magic.

Whip. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage, and the target is grappled (escape DC 12) and the unnamable horror cannot use this whip again. The horror has four whips.

Acid Spray (recharge 5-6). The unnamable horror sprays a stream of acid at one target creature within 30 feet. The target must succeed on a DC 14 Dexterity saving throw or take 6 (1d12) acid damage and an additional 4 (1d8) acid damage at the beginning of each of its turns. The target can make a DC 14 Constitution saving throw at the end of each of its turns, ending the effect on a success.