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

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

Sybiotic Cloak. Once per short rest the ooze can extend itself to just outside it's host and harden. During this period the creature gets a +2 to AC. The cloak lasts for 1 minute.

Kylaxian Leap. As a bonus action the creature can use psudopods to leap 10 feet. The first melee attack after this, on the same turn, get one extra damage die.

Kylaxian Regeneration. At the beginning of its turn the creature restores 10 hit points, unless struck with lightning damage the previous turn.

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 Large, 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 Large, it attains the maximum size possible in the space available.

Psyonic Hammer. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) bludgeoning damage plus 5 (1d10) psychic damage, or 15 (2d10 + 4) bludgeoning damage plus 5 (1d10) psychic damage while enlarged.

Dragon Fury Mortar. Ranged Weapon Attack:  +8 to hit, range 30/60 ft., one target. Hit: 12 (2d8 + 4). Upon a hit, everything within 5 ft of the target must make a Dexterity saving throw (DC 16) or suffer 1d8 fire damage, on a miss the round fails to detonate or rolls away harmlessly before doing so.. The mortar needs a bonus action to be reloaded after each shot. One an attack roll of 3 or lower the mortar misfires and requires an action to clear.

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.