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.

Multiattack. The elemental makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8+4) bludgeoning damage.

Engulf. The elemental moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces. Whenever the elemental enters a creature's space, the creature must make a DC 14 Dexterity saving throw.

On a successful save, the creature can choose to be pushed 5 feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.

On a failed save, the elemental enters the creature's space, and the creature takes 10 (3d6) necrotic damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 10 (3d6) necrotic damage at the start of each of the elemental's turns. When the elemental moves, the engulfed creature moves with it. Whenever the engulfed creature takes necrotic damage, the elemental gains a number of hit points equal to that amount.

An engulfed creature can try to escape by taking an action to make a DC 14 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the elemental.