Confusing Gaze. When a creature starts its turn within 30 feet of the necro hulk and is able to see the umber hulk’s eyes, the necro hulk can magically force it to make a DC 17 Charisma saving throw, unless the necro hulk is incapacitated.
On a failed saving throw, the creature can’t take reactions until the start of its next turn and rolls a d8 to determine what it does during that turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action but uses all its movement to move in a random direction. On a 7 or 8, the creature makes one melee attack against a random creature, or it does nothing if no creature is within reach.
Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can’t see the umber hulk until the start of its next turn, when it can avert its eyes again. If the creature looks at the umber hulk in the meantime, it must immediately make the save.

Tunneler. The necro hulk can burrow through solid rock at half its burrowing speed and leaves a 5 foot-wide, 8-foot-high tunnel in its wake.

Undead Fortitude. When the necro hulk is reduced to 0 hit points, it must make a constitution saving throw with a DC equal to half the damage taken, if it succeeds, it is resurrected with 50 hit points. The second time the necro hulk makes this throw, and every throw of this type afterwards, the DC is increased by +5.

Multiattack. The Necro Hulk makes three attacks, two with it's claws and one with its mandibles.

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 8) [slashing] damage. 

Mandibles. MeleeWeapon Attack: +10 to hit, range 10 ft., one target. Hit:  20(3d8 + 8) [slashing] damage.