Bone Hulk

The Grasping Hands.  The Bone Hulk's presence calls to any and all bones in the earth as hands, claws, ribs, and jaws rise to the surface and claw at the Bone Hulk's foes.  Any non-undead creature that starts its turn touching the ground within 10 feet of a Bone Hulk must succeed on a DC 15 Dexterity saving throw or take 14 (4d6) piercing damage and have its movement speed halved until the end of its turn. 

Undead Fortitude.  If damage reduces the Bone Hulk to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage dealt, unless the damage is thunder or from a critical hit.  On a success, the Bone Hulk drops to 1 hit point instead.

Unusual Nature.  The Bone Hulk doesn't require air, food, drink, or sleep.

Multiattack. The Bone Hulk makes two smash attacks, or uses its Bone Cage ability and makes one smash attack.

Smash. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage and 9 (2d6 +2) slashing damage. 

Bone Cage (Recharge 5-6).  The Bone Hulk either flings a mass of bone spears at a creature it can see within 40 feet of it, or calls the spears up from the ground underneath it.  The target must succeed on a DC 16 Dexterity saving throw or take 18 (4d8) slashing damage, and--if it is Large or smaller--be trapped by the bone spears.  A creature that succeeds on its save takes half damage and is not trapped by the cage.

A creature trapped by the cage is restrained, has three-quarters cover against all effects outside the cage, and takes 10 magical piercing damage at the start of each of its turns as tendrils of bone grow from the cage and burrow into its flesh.  The trapped creature is freed if the bone cage is destroyed.  The bone cage is a Large object with AC 10, 25 hit points, immunity to poison and psychic damage, resistance to necrotic damage, and vulnerability to thunder damage.