Shapechanger. The Xathak can use its action to polymorph into any creature, object, or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn 't transformed. It reverts to its true form if it dies. Additionally, when in it's polymorphed form it is indistinguishable from the creature whose form it has taken, to the point where not even magic can identify the Xathak as a fake.

Magical Resistance. The Xathak has advantage on all saving throws against spells and other magical effects.

Regeneration. The Xathak regenerates 19 hit points at the start of its turn. If the Xathak takes acid or fire damage, this trait doesn't function at the start of the Xathak's next turn. This trait also doesn't function if the Xathak takes the Amorphous Defence reaction. The Xathak only dies if it starts its turn with 0 hit points and doesn't regenerate.

Multiattack. The Xathak uses its Fearful Shriek if possible. It then makes three attacks: two with its tentacles and one with its bite.

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 20 (2d10 + 8) Piercing damage and the target must succeed on a DC 18 Constitution Saving Throw or be poisoned until the end of its next turn. 

Tentacle. Melee Weapon Attack: +15 to hit, range 15 ft., 1 target. Hit: 18 (2d8 + 8) Bludgeoning damage and must succeed on a DC 18 Constitution Saving Throw or take 16 (4d6) Poison damage.

Fearful Shriek (Recharge 5-6). The Xathak lets out an ear shattering shriek. Any creature within 120 ft. of the Xathak must succeed on a DC 21 Wisdom Saving Throw or be frightened until the end of its next turn.

Amorphous Defence (Recharge 4-6). The Xathak tears a hole in its form to avoid damage. When the Xathak is targetted by a physical or magical attack that would hit it, it can use its reaction to tear a hole in its body to defend from the attack.The damage from the triggering attack is negated, and it gains a +3 Bonus to AC until the start of its next turn.