If I move Sett into a battlefield to start a showdown, then during the showdown I play Zenith Blade to move Leona (Determined) to that battlefield, does Leona's 'When I attack' trigger activate, or did she need to be present when the showdown started?

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Accepted Answer

If Sett started the Showdown by moving to the battlefield, a unit moved there later by Zenith Blade was not present when Attacker/Defender were established. That unit will not be an Attacker at the moment the Showdown/Combat opened; it only gains the Attacker designation during the Cleanup after the action that moved it [459.2.b.3.a]. Therefore a "When I attack" effect that requires the unit to be an Attacker at the moment Combat opens will not trigger. If the ability instead triggers when the unit gains the Attacker designation, it would trigger when the unit gains that designation during the Cleanup — but the exact result depends on the precise wording of the unit's ability (which is not provided here).

Because the Attacker/Defender designations are established when the Combat/Showdown opens, a unit that arrives afterwards (moved there by Zenith Blade during the Showdown) does not become an Attacker immediately. It only gains the Attacker designation in the Cleanup following the action that caused it to become present [459.2.b.3.a]. So a "When I attack" trigger that requires the unit to have been present and an Attacker at the time the showdown/combat opened will not activate. If the trigger instead reads or functions as "When I become Attacker" and thus triggers on the moment the unit gains Attacker during Cleanup, that wording could allow it to trigger — but we can’t confirm without the exact card text (see assumptions). [459.2.b.3.a] [459.2.b.3] [459.2.b.1]

You move Sett to the opponent's battlefield; Contested is applied and a Showdown is opened per [446] and [344]. When the Showdown (and, if applicable, Combat) opens, the Attacker player and Attacker/Defender designations are established; Units already present controlled by the Attacker gain the Attacker designation now [459.2.b.1] [459.2.b.3]. You then play Zenith Blade in the Showdown to move Leona to that battlefield. Because Leona became present after Attacker/Defender were established, she does not become an Attacker immediately; she instead gains the Attacker designation during the Cleanup following the action that moved her [459.2.b.3.a]. If the Leona's "When I attack" ability requires she was an Attacker at the moment Combat opened, it will not trigger. If it triggers specifically when the unit gains the Attacker designation, it would trigger when she gains that designation during Cleanup — you must check the unit's exact wording.

If the Showdown had not yet opened (for example, the move that applied Contested staged a Showdown to be opened in cleanup but Combat/Open State timing differs), the timing of when Attacker/Defender are established could change the result; see [344]. If another effect changes control or re-establishes Attacker/Defender after Leona is moved (for example, another unit moves or an effect re-applies Contested), the moment she becomes Attacker could differ. If the unit's ability uses wording like "When I become Attacker" or "When I join an attacking force" instead of "When I attack (as the battle opens)", that specific wording may cause the ability to trigger when she gains Attacker during Cleanup — this ruling depends on the exact card text.

When a Showdown (and Combat if applicable) opens, the game establishes who is the Attacker and who is the Defender and gives Attacker/Defender designations to units that are present at that moment [459.2.b.1] [459.2.b.3]. The rules explicitly handle units that arrive after that moment: "If a Unit controlled by the Attacker becomes present at this Battlefield after this moment, it will gain the Attacker designation during the Cleanup phase following the action that caused it to become present" [459.2.b.3.a]. In your sequence, Sett's arrival opened the Showdown and established Attacker/Defender. Playing Zenith Blade later to move Leona into the battlefield makes her arrive after those designations were set, so she does not become an Attacker immediately — she gains that designation in the Cleanup after the move [446] [344] [459.2.b.3.a]. Many "When I attack" triggers check for the unit being an Attacker at the moment Combat opens; those will not see Leona as an Attacker if she arrived after the Showdown opened. If the specific Leona ability instead triggers when the unit "becomes" an Attacker (i.e., triggers on the moment the unit gains the Attacker designation), then it would trigger when she is given that designation during Cleanup. Because the exact Leona ability text you asked about is not included in the provided card data, I cannot conclusively say which of those two wordings applies to your Leona — the most defensible ruling from the Core Rules is that arriving after Showdown open means she was not an Attacker at open, and only gains Attacker during Cleanup, so only triggers that respond to gaining Attacker at that later time could fire [459.2.b.3.a] [459.2.d].