Xantcha, Sleeper Agent

{1}{B}{R}Legendary Creature — Phyrexian Minion5/5

Xantcha enters under the control of an opponent of your choice.

Xantcha attacks each combat if able and can't attack its owner or planeswalkers its owner controls.

{3}: Xantcha's controller loses 2 life and you draw a card. Any player may activate this ability.

Xantcha's controller may activate its last ability. In this case, the same player loses 2 life and draws a card. (2018-07-13)
Xantcha's first ability is a replacement effect that modifies how it enters the battlefield, not a triggered ability. Players can't take actions (such as activating its last ability) while Xantcha's on the battlefield before it's controlled by another player. (2018-07-13)
The player who draws a card when Xantcha's last ability resolves is the player who activated the ability. (2018-07-13)
If a player creates a token that's a copy of Xantcha, the player who creates the token is its owner, not the player under whose control it enters the battlefield. This is a change from previous rules. (2018-07-13)
If Xantcha can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either. (2018-07-13)
In a multiplayer game, if Xantcha's owner leaves the game, Xantcha leaves the game with them. If Xantcha's controller leaves the game, Xantcha is exiled. This is a change from previous rulings due to Xantcha's updated wording. (2019-08-23)