Capillary of D’Thuul

Amphibious. The Capillary of D’Thuul can breathe both air and water.

D’Thuulian Poison. A creature that is afflicted with D’thuulian poison cannot regain hitpoints or hit dice from resting until it is cured. After completing a long rest, the creature must succeed on a Constitution saving throw equal to the initial DC (11), taking 7 (1d12) poison damage and gaining a level of exhaustion on a failure, or half as much damage and no exhaustion on a success.

Regenerative. The Capillary of D’Thuul regains 6 hitpoints at the start of its turn, unless it has taken lightning damage

Multiattack. The Capillary of D’Thuul makes two tentacle attacks and one with its bite.

Tentacle. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage and 6 (1d12) acid damage 

Bite. Melee Weapon Attack: +7 to hit, 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage and 9 (2d8). Target must succeed on a DC 11 Constitution saving throw or become poisoned for 12 seconds (2 rounds) and afflicted with D’Thuulian Poison after 1 hour.

Retaliate. The Capillary of D’thuul makes one Tentacle or Bite as an opportunity attack as a reaction to being damaged by an attack.