Infernal Legacy. Xaalak knows the thaumaturgy cantrip, hellish rebuke (2nd) once per long rest, and can cast darkness once per long rest. CHA is his spell casting ability.

Potent Cantrips. Xaalak can add his spellcasting ability modifier to the damage he deals with any cantrip.

Spellcasting. Xaalak is an 8th-level warlock. His spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). Xaalak has the following warlock spells prepared:

Cantrips (at will): thaumaturgy, eldritch blast +3 damage, lightning lure +3 damage, prestidigitation

1st level (4 slots): arms of hadar, charm person, create or destroy water

2nd level (3 slots): darkness, hellish rebuke, misty step, mirror image

+2 Rapier. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 4) piercing damage.

Tentacle of the Deeps. (3/day) Xaalak can magically summon a spectral tentacle that strikes at his foes. As a bonus action, he creates a 10-foot-long tentacle at a point he can see within 60 feet. The tentacle lasts for 1 minute or until this feature is used to create another tentacle.

When Xaalak creates the tentacle, he can make a melee spell attack against one creature within 10 feet of it. On a hit, the target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of his next turn. 

As a bonus action on his turn, Xaalak can move the tentacle up to 30 feet and repeat the attack. He can summon the tentacle a number of times equal to his proficiency bonus, and regains all expended uses when on a long rest.

Parry. Xaalak adds 2 to his AC against one melee attack that would hit him. To do so, he must see the attacker and be wielding a melee weapon.