Montaron and the Laughing Skull

During his first turn, Montaron has advantage on attack rolls against any creature that hasn't taken a turn. Any hit Montaron scores against a surprised creature is a critical hit.

If Montaron is subjected to an effect that allows him to make a Dexterity saving throw to take only half damage, Montaron instead takes no damage if he succeeds on the saving throw, and only half damage if he fails.

Montaron can move through the space of any creature that is of a size larger than his.

Montaron deals an extra 21 (6d6) damage when he hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of Montaron that isn't incapacitated and Montaron doesn't have disadvantage on the attack roll.

Montaron possesses a bag of holding. He keeps the Demilich, Xzar, in the bag. He also possesses a portable hole, which he uses to aid in robberies. As a bonus action, Montaron can place his portable hole in his bag of holding. This instantly destroys both items and opens a gate to the Astral Plane. Montaron and any creature within 10 feet of him is sucked through the gate to a random location on the Astral Plane. The gate then closes.

Montaron makes three Shortsword attacks and uses the ability Unleash the Demilich if available.

Melee Weapon Attack +9 to hit, reach 5 ft., one target. 8 (1d6 + 5) Piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) Poison damage on a failed save, or half as much damage on a successful one.

Ranged Weapon Attack +9 to hit, range 80/320 ft., one target. 9 (1d8 + 5) Piercing damage.

Montaron reaches inside his bag of holding and releases Xzar. Xzar is a Demilich and appears within 5 feet of Montaron. Xzar rolls for his own initiative. Xzar can't use lair actions or legendary actions. He has a CR of 14.