It's as easy as taking candy from a baby. A giant, reptilian, firebreathing baby.

Agent of Treachery

When this creature enters, gain control of target permanent.
At the beginning of your end step, if you control three or more permanents you don't own, draw three cards.

Secret Lair Drop

"The more secrets you have, the more easily you become my pawn."

Core Set 2020

When Agent of Treachery enters the battlefield, gain control of target permanent.
At the beginning of your end step, if you control three or more permanents you don't own, draw three cards.

Core Set 2020 Promos

A token’s owner is the player who created it.

Agent of Treachery’s effect lasts indefinitely. It doesn’t wear off during the cleanup step, and it doesn’t expire if Agent of Treachery leaves the battlefield. In a multiplayer game, it does expire if you leave the game.

Gaining control of a creature doesn’t cause you to gain control of any Auras or Equipment attached to it. Gaining control of an Aura or Equipment doesn’t change what it’s currently attached to.

If you don’t control three permanents you don’t own as your end step begins, Agent of Treachery’s last ability doesn’t trigger. If you no longer control three permanents you don’t own as it resolves, you won’t draw three cards. Those don’t have to be the same three permanents each time.

If you gain control of a permanent “until end of turn,” that effect will wear off during the cleanup step, after the end step. You’ll control that permanent while considering Agent of Treachery’s last ability.