Blood Elemental

Sanguine Sense. While the blood elemental isn’t blinded, it can see any creature that isn’t an Undead or a Construct within 60 feet of itself, even through total cover, heavily obscured areas, invisibility, or any other phenomena that would prevent sight.

Necrotic Absorption. Whenever the elemental is subjected to necrotic damage, it takes no damage and instead regains a number of hit points equal to the necrotic damage dealt.

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 blood elemental makes two Blood Lash attacks.

Blood Lash. Melee Spell Attack: +8 to hit, reach 15 ft., one target. Hit: 14 (2d10 + 3) necrotic damage. If the target is a creature, it can’t regain hit points until the start of the blood elemental’s next turn.

Blood Boil (Recharge 6). The blood elemental chooses a point within 150 feet of itself, and a 20-foot radius sphere centered on that point fills with a burst of searing, blood-red mist. Each creature of the blood elemental’s choice that it can see in that area must make a DC 16 Constitution saving throw. On a failed save, a creature takes 28 (8d6) necrotic damage and is incapacitated until the end of its next turn. On a success, a creature takes half as much damage and isn’t incapacitated. A creature dies if reduced to 0 hit points by this necrotic damage.