My opponent played a Nova Chaser, championing out a Flamekin Harbinger. Now I want to steal his Nova Chaser by playing Agent of Treachery. When the Nova Chaser changes sides do i have to sacrifice it because I can't champion an elemental, and does his Flamekin Harbinger come back into play? Or does the Flamekin Harbinger still count as the champion even though it changed sides, meaning i can still play the Nova Chaser? Thanks for the help!
My opponent played a Nova Chaser, championing out a Flamekin Harbinger. Now I want to steal his Nova Chaser by playing Agent of Treachery. When the Nova Chaser changes sides do i have to sacrifice it because I can't champion an elemental, and does his Flamekin Harbinger come back into play? Or does the Flamekin Harbinger still count as the champion even though it changed sides, meaning i can still play the Nova Chaser? Thanks for the help!
You don't "play" the Nova Chaser, you just move it to your side of the battlefield as the Agent of Treachery's triggered ability resolves. The Nova Chaser doesn't enter the battlefield (or otherwise change zones) at that time, so the champion ability doesn't trigger.
400.1. A zone is a place where objects can be during a game. There are normally seven zones: library, hand, battlefield, graveyard, stack, exile, and command. Some older cards also use the ante zone. Each player has their own library, hand, and graveyard. The other zones are shared by all players.
702.71. Champion
702.71a Champion represents two triggered abilities. “Champion an [object]” means “When this permanent enters the battlefield, sacrifice it unless you exile another [object] you control” and “When this permanent leaves the battlefield, return the exiled card to the battlefield under its owner’s control.”
Enters the Battlefield
A nontoken permanent “enters the battlefield” when it’s moved onto the battlefield from another zone. A token “enters the battlefield” when it’s created. See rules 403.3, 603.6a, 603.6d, and 614.12.
If the Nova Chaser leaves the battlefield while under your control, the Elemental it championed will still return under its owner's control.
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Why bother with mere rulings when so many answers can be found in the Rules?
Champion a ____ is an enter the battlefield ability and a leave the battlefield ability. Changing control does not trigger enter the battlefield abilities nor leave the battlefield abilities because nothing is entering or leaving the battlefield, it is just changing control.
You are not playing the Nova Chaser and it is not entering the battlefield, it stays on the battlefield and simply changes controller. Flamekin Harbinger remains exiled. To note, if Nova Chaser leaves the battlefield while it was under your control, the Harbinger still returns to the battlefield under its owner's control.
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
You don't "play" the Nova Chaser, you just move it to your side of the battlefield as the Agent of Treachery's triggered ability resolves. The Nova Chaser doesn't enter the battlefield (or otherwise change zones) at that time, so the champion ability doesn't trigger.
If the Nova Chaser leaves the battlefield while under your control, the Elemental it championed will still return under its owner's control.