Blood Elemental

Blood Form. The elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Nauseating Blood. When a creature enters a space occupied by the blood elemental or starts its turn there, it must succeed on a DC 15 Constitution saving throw or be poisoned for 1 minute. The creature can attempt this saving throw again at the end of each of its turns. Once a creature has succeeded on this  save it is immune to the effects of the elemental’s nauseating blood for 24 hours.

Blood Drain. Whenever the elemental deals necrotic damage to a target that has blood it gains a number of temporary hit points equal to half the damage dealt and the creature’s maximum hit points are reduced by the amount of necrotic damage they take until they finish a long rest.

Multiattack. The elemental makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 11 (2d6 + 4) bludgeoning damage and 7 (2d6) necrotic damage.

Bloodbath (Recharge 5-6). Each creature in the elemental's space must make a DC 15 Strength saving throw. On a failure, a target takes 11 (2d6 + 4) necrotic damage. If it is Large or smaller, it is also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breathe. If the saving throw is successful, the target is pushed out of the elemental's space.

Cause Insanity (Recharge 6). The elemental assaults and twists the mind of a single creature it can see within 60 feet, spawning delusions and provoking uncontrolled action. The target is subjected to the confusion spell (DC 15 Wisdom saving throw). If the target succeeds on any saving throw against this effect, it is immune to the elemental’s Cause Insanity action for 24 hours.