Crown of Thieves
Traits
Evasion.
Legendary Resistance (2/Day).
Sneak Attack (1/Turn). 35 (10d6) extra damage.
Elusive. Attack rolls against the Crown of Thieves never have Advantage.
Shadow Guild. As long as it remains unseen, allies within 60 feet attack with Advantage.
Actions
Multiattack. Makes four Crownblade attacks.
Crownblade. Melee Attack Roll: +13, reach 5 ft. Hit: 14 (2d6 + 7) Slashing damage plus 10 (3d6) Poison damage.
Crown's Decree (Recharge 5–6). The Crown of Thieves targets three creatures it can see within 60 feet. Each makes a DC 21 Dexterity save. Failure: 45 (10d8) Piercing damage (blades leaping from the shadows) and Prone. Success: Half damage.
Bonus Actions
Cunning Action. Dash, Disengage, or Hide.
Shadow Slip. Teleports to a point it can see within 60 feet that is in Dim Light or Darkness.
Reactions
Uncanny Dodge. Halves the damage.
Deflect Missile. When hit by a ranged attack, reduces the damage by 1d10 + 7 + 17. If the damage is reduced to 0, it can catch the missile and throw it back (Ranged Attack Roll: +13, range 20/60, 14 (2d6+7) damage).
How to Use
King of the underworld. A Tier 4 urban boss.
Elusive is very strong: you cannot gain Advantage. A Rogue's Sneak Attack, Pack Tactics, Faerie Fire — none of it works.
Shadow Slip (Bonus Action teleport) plus Cunning Action (Disengage) means it's never caught.
Key
Bright Light (Shadow Slip requires Dim Light/Darkness!). In a lit arena the Crown of Thieves can't teleport. This completely changes the fight.