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

Psychic Conquest. Xardorok deals an extra 5 (1d10) psychic damage to any creature he hits with a weapon attack, and he regains a number of hit points equal to the amount of psychic damage dealt.

Innate Spellcasting. Xardorok’s innate spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

At will: eldritch blast, mage hand

2/day each: hold person, suggestion

1/day: cloudkill

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

Legendary Resistances (2/round). 

• The creature immediately makes a saving throw against every effect currently affecting them. This includes effects that don’t have repeat saving throws, such as bane or banishment, but not effects that do not have saving throws.
• The creature gains resistance to all damage dealt from a single attack. 
• The creature can expend hit dice to immediately recover health equal to the number rolled. If the creature has no more hit dice, or if they are prevented from regaining HP, they cannot use this resistance

Multiattack. Xardorok attacks twice with a weapon or casts eldritch blast twice.

Spiked Gauntlet. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 6) piercing damage, or 10 (2d8 + 6) piercing damage while Xardorok is enlarged plus 5 (1d10) psychic damage. He regains a number of hit points equal to the amount of psychic damage dealt. If the target is a Large or smaller creature, it must make a successful DC 17 Strength saving throw or be thrown up to 30 feet away in a straight line. The target lands prone and then takes 10 (3d6) bludgeoning damage.

Stomping Foot. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) bludgeoning damage, or 18 (3d8 + 5) to a prone target, or (2d8+5) and (4d8+5) respectively if he is enlarged.

Eldritch Blast (Cantrip). Ranged Spell Attack: +9 to hit, range 120 ft., one creature. Hit: 10 (1d10 + 6) force damage.

Invisibility (Recharge 4–6). Xardorok magically turns invisible until he attacks, he casts a spell, he uses his Enlarge, or his concentration is broken (as if concentrating on a spell). Any equipment Xardorok wears or carries is invisible with him.

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

Warp Space (Recharge 4-6). Xardorok bends space to his will, allowing him to teleport to an unoccupied space that he can see within 120ft. Alternatively, he can teleport a creature that he can see within 120ft to an unoccupied space that he can see within range. If the creature is unwilling, it must make a DC 17 Charisma Saving Throw or be teleported.

Hellish Rebuke (2/Day). When Xardorok is damaged by a creature within 60 feet of him that he can see, the creature that damaged him is engulfed in hellish flames and must make a DC 17 Dexterity saving throw, taking 16 (4d10) fire damage on a failed save, or half as much damage on a successful one.

Psychic Break (2/day). When Xardorok sees a spell cast within 60ft of him, he can send a psychic burst to interrupt the casting. The creature casting the spell must make a DC 17 Intelligence saving throw, taking 2d8 psychic damage on a failed save and must make a concentration check or lose their spell.

Xardorok can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Xardorok regains spent legendary actions at the start of his turn.

At-Will Spell. Xardorok casts one of his at-will spells.

Melee. Xardorok attacks with his Spiked Gauntlet or with his Stomping Foot.

Mind Mastery (2 Actions). Xardorok targets one creature it can see within 60 feet of it. The target must succeed on a DC 17 Intelligence saving throw, or the duergar causes it to use its reaction, if available, either to make one weapon attack against another creature the duergar can see or to move up to 10 feet in a direction of the duergar’s choice. Creatures that can’t be charmed are immune to this effect.

Psychic Grasp (2 Actions). Xardorok reaches out at a creature within 60ft. That creature makes a DC 17 Strength Saving Throw. A Creature takes 5d8 bludgeoning damage and is thrown up to 60ft away in any direction and is knocked prone, or half as much on a successful save and isn't thrown.

Psionic Burst (3 Actions). Xardorok releases an intense burst of psychic energy. Each creature in a 20-foot-radius sphere centered on Xardorok must make a DC 17 Intelligence saving throw. A creature with an Intelligence score of 2 or lower can’t be affected by this spell. A target takes 8d6 psychic damage on a failed save, or half as much damage on a successful one.

After a failed save, a target has muddled thoughts for 1 minute. During that time, it rolls a d6 and subtracts the number rolled from all its attack rolls and ability checks, as well as its Constitution saving throws to maintain concentration. The target can make an Intelligence saving throw at the end of each of its turns, ending the effect on itself on a success.