Force Sense. The Xenosith has truesight and can sense the exact location of any creature, construct, or undead within 90 feet.  

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

Acid Blood. Any creature within 5 feet of the xenosith when it takes damage, other than bludgeoning damage, takes 2d8+6 acid damage.

Telekinetic Sword Fighting. The Xenosith uses a bonus action to throw it's lightsaber into the air, within 30 feet of it. The lightsaber attacks twice using the xenosith's attack modifier, and can move up to 30 feet. This lasts until the xenosith breaks concentration like concentrating on a spell, or until the xenosith ends it as a free action.

Hyperreactive. The Xenosith has 2 reactions. 

Force Scream. When the Xenosith drops below 100 health it automatically casts the  psychic scream, without using a spell slot. This ability recharges on a long rest.

Legendary Resistance (5/Day). If the Xenosith fails a saving throw it can choose to succeed instead.

Frightful Presence. Each creature of the xenostih's choice that is within 120 feet of the xenosith and aware of it must succeed on a DC 26 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the xenosith's Frightful Presence for the next 24 hours.

Regeneration. The xenosith regains 15 hit points at the start of its turn. If the xenosith takes necrotic damage, this trait doesn't function at the start of the xenosith's next turn. The xenosith dies only if it starts its turn with 0 hit points and doesn't regenerate.

Spellcasting. The Xenosith is a 20th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 25, +17 to hit with spell attacks). The xenosith has the following spells prepared:

Cantrips (at will):  invisibility,  lightning bolt,  pulse wave, counterspell

5th level (4 slots): hold monster,  destructive wave, dominate person, telekinesis

6th level (3 slots):  gravity fissure,  mental prison,  chain lightning

7th level (3 slots):  plane shift, regenerate

8th level (2 slots): dark star,  mind blank*

9th level (2 slots): foresight*, psychic scream

* the xenosith casts these spells on itself before combat. 

Multiattack. The xenosith makes 4 attacks: 3 lightsaber attacks and one tail attack, or two with its claws, one with its tail, and one bite.

Lightsaber. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 10 (2d10 + 10) force damage. (see "Xenosith's Lightsaber" for added effects.)

Lightsaber Throw. The xenosith throws it's lightsaber in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 24 Dexterity saving throw, taking 61 (6d10+30) force damage on a failed save, or half as much damage on a successful one. The lightsaber returns after this. 

Claw. Melee Weapon Attack: +16{"diceNotation":"1d20+16","rollType":"to hit","rollAction":"Claw"} to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) slashing damage. 

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 7) piercing damage plus 7 (2d6) acid damage.

Tail. Melee Weapon Attack: +16 to hit, reach 10 ft., one creature. Hit: 15 (3d10 + 7) piercing damage. If the target is Large or smaller, it is grappled (escape DC 24). Until this grapple ends, the target is restrained, the Xenosith can automatically hit the target with its tail, and the Xenosith can't make tail attacks against other targets.

Force Destruction. (Recharge 5–6). The Xenosith draws on dark powers of the force to destroy everything in front of it in a 60-­foot line that is 15 feet wide. Each creature in that line must make a DC 25 Dexterity saving throw, taking 72 (8d12+20) force damage on a failed save, or half as much damage on a successful one. Creatures reduced to zero hit points are turned to ash.

Telekinetic Sword Fighting. The Xenosith uses a bonus action to throw it's lightsaber into the air, within 30 feet of it. The lightsaber attacks twice using the xenosith's attack modifier, and can move up to 30 feet. The Xenosith can use a bonus action on subsequent turns to move the lightsaber an additional 30 feet and attack while this effect lasts. This lasts until the xenosith breaks concentration like concentrating on a spell, or until the xenosith ends it as a free action.

Quickened Spell. All spells cast by the Xenosith take a bonus action to cast.

Destroy Weapons. The Xenosith can parry while attuned to it's lightsaber. As a reaction add 3 to your AC against a single melee attack that would hit you. Any nonmagical weapon that hits the Xenosith's lightsaber is damaged. After dealing damage, the weapon takes a permanent and cumulative −2 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. This doesn't effect natural weapons.

Counterspell. The Xenosith can cast the spell counterspell.

The xenosith 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. The xenosith regains spent legendary actions at the start of its turn.

Tail Attack. The xenosith makes a tail attack.

Lightsaber Attack. The xenosith makes a lightsaber attack.

Cast a Spell. The xenosith casts a spell.

 Quick Step. The xenosith moves up to its speed without provoking opportunity attacks.