Shapechanger. The Shapeshifter can use its action to polymorph into a creature of any size as long as it has a source of light, or back into its true form. Its statistics, other than its size, 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.

Ambusher. The Shapeshifter has advantage on attack rolls against any creature it has surprised.

Multiattack. Xarr makes two shadow claw attacks and then if possible uses a stone attack.

Shadow Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.                         Hit: 46 (16d6 + 4) necrotic damage.

Absorb Blood. Ranged Weapon Attack: +13 to hit, reach 20 ft., one target. 35 necrotic damage. heals shapeshifter 35 hp.

Future. Stone Attack. +15 to hit, one target. decrease one of the target's stats by one. 3 charges.

 Present. Stone Attack. +15 to hit, one target. skip target's next turn. 2 charges.

Past. Stone Attack. +15 to hit, one target. decrease target's level by one.

Parry. Xarr reduces his damage taken by 3d6 if he is aware of the attack.

Xarr can use 3 legendary actions per turn.

Shadow Claw.  Xarr makes a shadow claw attack.

Absorb Blood (Costs 2 actions) Xarr makes an absorb blood attack. 

Chronon Stone Spell (Costs 3 actions) Xarr casts either heal (on himself), finger of death, or greater invisibility. Xarr can not use legendary actions on his next turn.

Burst of Darkness (Costs 3 actions) Xarr adds 2 to his AC until the end of his next turn. Players must succeed on a DC 13 Con. Saving Throw or be blinded. They can repeat the saving throw at the start of each of their turns.