Confusing Gaze. When a creature starts its turn able to see the umber hulk's eyes within 30ft, the umber hulk can magically force it to make a DC13 Cha save unless it 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.  1-4: do nothing.  5-6: no action, but uses all its movement in a random direction.  7-8: the creature makes one melee attack against a random creature or does nothing if no creature is within reach.
Unless surprised, a creature can avert its eyes to avoid the saving throw.  If it does so, it can't see the umber hulk until the start of its next turn.

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

Multiattack. The umber hulk makes two attacks: one with its claws and one with its  claw or its mandibles.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 6 (1d6 + 3) slashing damage. 

Mandibles. MeleeWeapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 9 (2d6 + 3) slashing damage.