Magic Resistance. The Xillathiad has advantage on saving throws against spells and other magical effects.

Innate Spellcasting (Psionics). The Xillathiad's innate spellcasting ability is Intelligence (spell save DC 15). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate

1/day each: dominate monster

Acidic Blood. A Xillathiad has a caustic blood like fluid that flows beneath its carapace.  A creature that deals bludgeoning, piercing, or slashing damage to the xenomorph while standing within 5 feet of it must make a DC 15 Dexterity saving throw, taking 14 (4d6) acid damage on a failed saving throw, or half as much acid damage on a successful one.  Additionally any creature that failed its save takes half as much damage on the following round.

Echolocation. The Xillathiad can’t use its blindsight while deafened.

Pack Tactics. The Xillathiad has advantage on an attack roll against a creature if at least one of the Xenomorph’s allies is within 5 feet of the creature and the ally isn’t incapacitated.

Spider Climb.  The Xillathiad can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Darkness Camouflage. The Xillathiad has advantage on all Stealth checks when in Dim Light or Darkness

Mind Blast (Recharge 5–6). The Xillathiad magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

MultiAttack. A Xillathiad makes four attacks: one with its tentacles, two with its claws, and one with its tail. 

Tentacles (Bite). Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain (Bite). Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the Xillathiad. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the Xillathiad kills the target by extracting and devouring its brain.

Claws. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit:  8 (2d4 + 4) slashing damage. Any creature hit by the Xillathiad's claws must make a DC 15 Dexterity check or be grappled.

Tail. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 11 (2d6 + 4) Piercing damage.