Board & Card Games Stack Exchange is a question and answer site for people who like playing board games, designing board games or modifying the rules of existing board games. It only takes a minute to sign up.
Sign up to join this community
Xantcha, Sleeper Agent has the ability:
As Xantcha, Sleeper Agent enters the battlefield, an opponent of your choice gains control of it.
Warstorm Surge has the ability:
Whenever a creature enters the battlefield under your control, it deals damage equal to its power to any target.
If I cast a Xantcha, my opponent will ultimately gain control of it. But is it ever on the battlefield under my control, such that it will trigger my Warstorm Surge?
No, it enters the battlefield under the opponent's control and is not even momentarily under your control. If you both had Warstorm Surges, it would trigger theirs and not yours.
"As (this card) enters the battlefield" is a replacement effect that altogether changes how it would enter the battlefield. In this case, it replaces the card entering under your control with entering under theirs. This is like how a Mistcutter Hydra does not enter the battlefield as a 0/0, then gain +1/+1 counters (it would die) — it enters with those counters and, from the rules' perspective, had them all along for as long as it was on the battlefield.
We can check this in the rules for replacement effects:
614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . “ are replacement effects.
It would trigger your Warstorm Surge (and not an opponent's) if it instead said:
When Xantcha, Sleeper Agent enters the battlefield, an opponent of your choice gains control of it.
... because that is a triggered ability that would have to go on the stack and resolve first, after Xantcha enters under your control.
Xantcha will enter the battlefield under your opponent's control, not your control, so she will not trigger your Warstorm Surge.
Xantcha's quoted ability is a replacement effect that modifies how she enters the battlefield, so it applies before she enters the battlefield.
