Behemoth

Boulder Body. Behemoth is immune to difficult terrain

Living Fortress. While Behemoth is prone, any damage dealt to it is reduced by 11 (2d10). This ability has no affect on psychic and necrotic damage.

Final Stand. When Behemoth reaches 50 hit points, they become enraged, gaining advantage on attack rolls and strength saves

Multi-attack. Behemoth can perform two smash attacks, one of which can be replaced with the bite attack.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 20 (2d10 + 7) bludgeoning damage. The behemoth regains hitpoints equal to the damage dealt by this attack.

Smash. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 7) bludgeoning damage. 

Lava Bomb. Behemoth spews molten rock, hitting a point within 30 feet of the Behemoth. All creatures within 10 feet of that point must make a DC 18 dexterity throw or take 19 (2d8 + 7) bludgeoning and 38 (6d10) fire damage.

Rock Wall. Behemoth summons a non-magical wall of solid stone at a point of its choosing within 20 feet. The wall is 6 inches thick with a height of 30 feet and a width of 50 feet. If the wall cuts through a creatures space when it appears, the creature is pushed to the opposite side of the wall, taking 24 (3d10+7) bludgeoning damage.

Fissure (recharge 6) Behemoth smashes the ground with immense force, splitting the earth below itself. It causes the magma to erupt below all creatures within 40ft of itself. All creatures in this range must succeed a DC 22 dexterity saving throw, or take 82 (16d8) fire damage. The ground affected becomes difficult terrain.

Crush. If a creature moves below Behemoth, all creatures under Behemoth must succeed a DC 18 dexterity saving throw, taking 39 (5d10 + 7) bludgeoning damage. All creatures affected will be knocked prone, not being crushed again unless another creature moves below Behemoth.