Ambush

Can I use Ambush's Reaction timing at my base or a battlefield where I control no units?Copy Anchor Link

Not from the standard Ambush keyword alone: it grants the unit Reaction only while you are playing it to a battlefield where you control units.^{[822.1.b][813.4]} Normal unit-play permission makes your base a valid location,^[355.2.a] and Last Caress can make a battlefield where you control no units valid.^[355.2.b] Neither separate location permission expands Ambush or satisfies its condition for granting Reaction.

An ability can explicitly expand Ambush to other battlefields; when it does, Ambush can grant Reaction there.^[822.1.d] If the unit independently has or gains Reaction, you may instead rely on that separate timing permission.^[813.1.c.1]

Can opponents react to a unit being played with Ambush?Copy Anchor Link

No. Ambush changes when the unit can be played,^[822.1.b] but the unit otherwise enters normally. Finalizing it does not pass priority.^[337.1.a] It then resolves and enters the board immediately.^{[337.2][359.2.c]}

This is a common point of confusion with cards like Kha'Zix, Mutating Horror, which has Ambush and triggers when he attacks or defends if an enemy unit is alone at his battlefield. An opponent cannot play a unit in response to Kha'Zix being placed on the chain in an attempt to break the "alone" condition before he enters.

Who gets to act after I Ambush a unit during a showdown?Copy Anchor Link

After the Ambush unit enters, its entry causes a cleanup, during which priority and focus are not passed or awarded.^{[319.6][320.1]} After that cleanup, ask whether anything remains on the chain.

• Nothing remains on the chain: If the unit started the chain and no other item remains after cleanup, the chain closes,^[330] and the next player gains focus and priority.^{[347.1.b][313.2]}
• Another item remains on the chain: An ability triggered by the unit's entry can finalize and remain.^{[383.2.c][383.3][407.2]} An older item can remain if the unit was played in response to it.^{[337.2][340.1]} The newest remaining item's controller gains priority, and focus does not pass.^{[337.4][340.4][346]}