Barrak

Duergar Resilience. Barrak has advantage on saving throws against poison, spells, and illusions, as well as to resist being charmed or paralyzed.

Sunlight Sensitivity. While in sunlight, Barrak has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Enlarge (Recharges after a Short or Long Rest). For 1 minute, the duergar magically increases in size, along with anything it is wearing or carrying. While enlarged, the duergar is Huge, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the duergar lacks the room to become Huge, it attains the maximum size possible in the space available.

Invisibility (Recharges after a Short or Long Rest). The duergar magically turns invisible until it attacks, casts a spell, or uses its Enlarge, or until its concentration is broken, up to 1 hour (as if concentrating on a spell). Any equipment the duergar wears or carries is invisible with it.

Multiattack. Barrak makes two attacks with ether his tail or claws. 

Demon Claws. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage, or 13 (2d8 + 4) piercing damage while enlarged.

Tail Whip. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) slashing damage, or 9 (2d4 + 4) slashing damage while enlarged.

This creature has two legendary actions per round and they reset at the top of the round. It can spend one action to make an attack with its tail during any other  turn than its own.