Bua the abomination

Virulent Breath. Noxious Gas from Bua´s digestion of previous meals spews from its mouth. At the start of Bua's turn, each creature within 5 feet of it must succeed on a DC 15 Constitution saving throw or take 5  (1d10) poison damage.

Legendary resistance  (2/Day). When Bua fails a saving throw, she can choose to succeed instead.

Inmortal body. When Bua dies her body regenerates within the next 24 hours unless completely destroyed.

Multiattack. Bua makes two Bite or Tentacle attacks. It can replace one of these attacks with Chitin Slam

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw against disease or become poisoned until the disease is cured. Every 24 hours that elapse, the target must repeat the saving throw, reducing its hit point maximum by 5 (1d10) on a failure. The disease is cured on a success. The target dies if the disease reduces its hit point maximum to 0. This reduction to the target's hit point maximum lasts until the disease is cured.

Tentacle. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 4 (1d8) piercing damage. If the target is Medium or smaller, it is grappled (escape DC 15) and restrained until the grapple ends. Bua has two tentacles, each of which can grapple one target.

Chitin Slam Bua targets one creature grappled by her, slamming the creature against her chitinous plating. The creature must succeed on a DC 15 Constitution saving throw or take 16 (3d10) bludgeoning damage and be stunned until the end of Bua's next turn. On a successful save, the target takes half the bludgeoning damage and isn't stunned.

Liquid movement. Bua drops flat and partially buries herself onto the ground. Bua can move up to half her speed without provoking oportunity attacks; she can also move below a creature, if she does, the creature takes 3 (1d6) piercing damage. If Bua ends her movement under another creature's space, the creature gets pushed to the closest empty space.

Aberrant adaptability. Bua´s body has changed not only to evade death but to ward itself from harm. Inmediately before taking damage, Bua can become resistant to the chosen type of damage until the beginning of her next turn.