How does Rengar - Trophy Hunter work when the opponent passes focus on a battlefield where you have no units?

Question

Accepted Answer

You cannot use Ambush to play Rengar - Trophy Hunter as a Reaction on a battlefield where you have no units. The card’s extra sentence does allow you to play it to a battlefield that contains enemy units even if you control none, but Ambush only grants the Reaction timing when you are playing the unit to a battlefield where you control units. If your opponent’s passing caused the Showdown to close before you could play anything, you also lose the window to play it there.

If you have no units at that battlefield, you may play Rengar - Trophy Hunter to a battlefield that contains enemy units because of the card’s permission, but you do not get Ambush’s Reaction timing for that play. Ambush grants Reaction only while the unit is being played to a battlefield where you control units [822]. If all players pass and the Showdown closes before you play anything, the opportunity to play in that Showdown is lost [348].

You are at a battlefield where you have no units and your opponent has units there (so the battlefield contains enemy units). Your opponent 'passes focus' (i.e., passes priority during the Showdown window). If you still have an opportunity to act (you have priority/focus after their pass), you may attempt to play cards while the Showdown is open. To play Rengar - Trophy Hunter, the card’s text allows it to be played to a battlefield that contains enemy units even if you control none (this is a permission distinct from Ambush’s usual "where you control units" permission) [822.3]. However, Ambush only grants the special Reaction timing when the unit is being played to a battlefield where you control units. Because you control no units there, Ambush does not give Reaction timing for this play — the play would not be a Reaction provided by Ambush [822]. If instead all players pass (no one plays a spell or activates an ability) so the Showdown closes, the window to play in that Showdown is gone and you cannot play Rengar - Trophy Hunter there at that moment [348].

If another effect gives you control of a unit at that battlefield before you play or grants Rengar - Trophy Hunter Reaction timing, Ambush could grant Reaction (because Ambush’s Reaction depends on being played to a battlefield you control) [822]. If there are actually no units at the battlefield by the time Finalization completes for your play, the Ambush permission would fail and you cannot rely on Ambush to make that location legal; however the card’s separate sentence that it “can be played to a battlefield where there are enemy units” only helps when enemy units are present during finalization [822.3]. If the opponent’s pass is part of the sequence where all players pass and the Showdown closes immediately, you do not get an additional Reaction window after the Showdown closes to play the card there unless another rule or effect creates a new opportunity [348].

Short answer: playing Rengar - Trophy Hunter to a battlefield that only has enemy units is allowed by the card’s own text, but Ambush does not give you Reaction timing when you control no units at that battlefield. Ambush is functionally short for “I may be played to a battlefield where you control Units” and also grants the unit [Reaction] only while it is being played to a battlefield you control [822]. The printed extra sentence on Rengar - Trophy Hunter that it “can be played to a battlefield where there are enemy units (even if you don't have units there)” grants an additional location-permission, but it does not change Ambush’s rule that the Reaction timing only applies when the unit is being played to a battlefield you control [822]. If your opponent’s pass resulted in all players passing and the Showdown closing before you could play anything, that closes the window to play in that Showdown and you can’t play the card there at that time [348]. Finally, if the battlefield ends up with no units at the moment Finalization completes for the play, Ambush’s location permission would be invalidated and you could not rely on Ambush to play it there [822.3].