Thievery. On each of its turns, it makes a sleight of hand check contested with your strength check. On a successful check, it appears in front of you and steals your weapon

Spellcasting. The Thief is a  10th-level spellcaster. Its spellcasting ability is Intelligence spell save DC 15, +5 to hit with spell attacks. The Thief has the following spells prepared:
Cantrips: Mage hand, Prestidigitation, Minor Illusion, Shocking Grasp 

1st lvl Disguise self, Command, False Life and Hellish Rebuke

2nd lvl Invisibility, Rope Trick and Flaming Spear

3rd lvl Bestow Curse, Blink, Counterspell

4th lvl Polymorph, Death Ward and Dimension Door

5th lvl Geas, Cone Of Cold

Mutiattack. The Thief makes 3 attacks, one with an item it is holding and two with its claws, and can forgo any number of them to use thievery instead

Claw. +6 to hit, 2d6 slashing

Improvised Attack. +2 to hit, whatever weapon it is holding damage

Shadow Dodge. When a creature makes a melee attack with disadvantage, it automatically misses