Black Beast of Aaaaarrrrrrggghhh

Form of Dread. The Beast’s exact shape is difficult to comprehend. A creature that starts its turn within 30 feet of the Beast must succeed on a DC 16 Wisdom saving throw or have disadvantage on its next attack roll before the end of its turn.

Half-Seen Horror. While in dim light or darkness, the Beast has advantage on attack rolls and Dexterity (Stealth) checks.

Legendary Resistance (3/Day). If the Beast fails a saving throw, it can choose to succeed instead.

The Animator Suffered a Fatal Heart Attack. If the Beast rolls a natural 1 on an attack roll, saving throw, or ability check, it immediately drops to 0 hit points and dies.

Multiattack. The Beast makes two Claw attacks and one Bite attack.

Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 19 (3d8 + 6) [Slashing] damage.

Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 24 (3d10 + 6) [Piercing] damage. If Large or smaller, grappled (escape DC 18).

Disengage. If you take the disengage bonus action, your movement doesn't provoke opportunity attacks for the rest of the turn,

Sudden Loom. When a creature misses the Beast with a melee attack, the Beast may move up to 15 feet without provoking opportunity attacks.

The Black Beast of Argh can take 3 legendary actions, choosing from the options below:

Shadow Step. Teleport up to 30 feet to an area of dim light or darkness.

Claw. Make one Claw attack.