Corpse Behemoth

Aversion to Fire. If the Corpse Behemoth takes Fire damage, it has Disadvantage on attack rolls and ability checks until the end of its next turn.

Berserk. Whenever the Corpse Behemoth starts its turn Bloodied, roll 1d6. On a 6, the Corpse Behemoth goes berserk. On each of its turns while berserk, the Corpse Behemoth attacks the nearest creature it can see. If no creature is near enough to move to and attack, the Corpse Behemoth flails aggressively . Once the Corpse Behemoth goes berserk, it remains so until it is destroyed or it is no longer Bloodied or can no longer find a target.

Immutable Form. The Corpse Behemoth can’t shape-shift.

Magic Resistance. The Corpse Behemoth has Advantage on saving throws against spells and other magical effects.

Legendary Resistance. (3/Day, or 4/Day in Lair). If the Corpse Behemoth fails a saving throw, it can choose to succeed instead.

Multiattack. The Corpse Behemoth makes three claw attacks. And either one bite or Maul attack

Claw. Melee Attack Roll: +7, reach 5 ft. Hit: 13 (2d6 +4) Slashing damage plus 4 (2d6) Necrotic damage.

Bite. Melee Attack Roll: +8, reach 5 ft. Hit: 13 (1d12 + 6) Necrotic damage plus 6 (1d12) Necrotic damage.

Spine Maul. Melee Attack Roll: +11, reach 5 ft. Hit: 13 (4d6 + 6) Bludgeoning damage plus 6 [rollable](4d6);

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn , the Corpse Behemoth can expend a use to take one of the following actions. The Corpse Behemoth regains all expended uses at the start of each its turns.

Frightful Presence. Charisma Saving Throw: DC 15, creatures in a 60-foot-radius. Failure: and it has the Frightened condition (resist DC 15) to overcome. While Frightened, the target has the Frightened condition. Success: No Effect.

Corpse Pit. The Corpse Behemoth summons 1d4 undead enemies (consult the monster Manual for any appropriate undead creature), pupated by it's tendrils snaking under the surface of the blood pool in it's lair (DC, 15 Perception/Investigation check reveals the tendril) Tendril has an AC of 10 and has 1 hit point, destroying the tendril immediately kills the undead puppet) Once the creature is destroyed the tendril can no longer puppet it.

Gore Tendrils.  (Recharge 5–6). Dexterity Saving Throw: DC 15, upto 5 creatures in a 50-foot-radius. Failure: 4 (1d6) Necrotic damage. and it has the Grappled condition (escape DC 15) from all five tendrils. While Grappled, the target has the Restrained condition. If the Target is not within melee it is pulled 20ft towards the Corpse Behemoth. Success: Half damage.