Blood Leviathan

Legendary Resistance (3/Day). If the leviathan fails a saving throw, it can choose to succeed instead.

Siege Monster. The leviathan deals double damage to objects and structures.

Blood Form. The leviathan 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 leviathan makes a Slam attack and two Tail attacks.

Slam. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 21 (2d12+ 8) bludgeoning damage plus 13 (2d12) necrotic damage.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 19 (2d10 + 8) bludgeoning damage plus 10 (3d6) necrotic damage.

Blood Wave (Recharge 6). The leviathan magically creates a wave of Blood that extends from a point it can see within 120 feet of itself. The wave is up to 250 feet long, up to 250 feet tall, and up to 50 feet wide. Each creature in the wave must make a DC 24 Strength saving throw. On a failed save, a creature takes 45 (7d12) Necrotic damage and is knocked prone. On a successful save, a creature takes half as much damage and isn’t knocked prone. The Blood spreads out across the ground in all directions, extinguishing unprotected flames in its area and within 250 feet of it, and then it vanishes. The leviathan heals for half of the damage done