When Overzealous Fan's 'When I defend, you may kill me to move an attacking unit to its base' ability triggers, can the defending player wait to decide whether to use it after seeing the attacking player's reactions?

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Accepted Answer

Yes. When Overzealous Fan's defend trigger goes on the chain you do not have to immediately kill it — you can let players put Reactions on the chain first. The decision to kill Overzealous Fan (the optional cost inside the instruction) is made when that triggered ability's instruction executes as it resolves; if you choose to kill it at resolution and the kill and move are legal at that time, they happen. [355.10.c.1] [359.3.e.11] [359.3.e.5]

When Overzealous Fan's "When I defend, you may kill me to move an attacking unit to its base" triggers, the trigger is placed on the chain. Players may add Reactions to the chain before that trigger resolves. The choice to perform the "you may kill me" instruction is made as the trigger's instruction executes on resolution — it is an optional instruction (whose kill is the cost within that instruction) and is not required to be paid immediately when the trigger is placed on the chain. If, at resolution, you choose to kill Overzealous Fan and both the kill and the move are legal, you kill it and then move the chosen attacking unit to its base. If parts of the instruction are illegal at resolution (for example the attacking unit is no longer a legal target), those parts are ignored per the usual instruction/target legality rules. [355.10.c.1] [359.3.e.3] [359.3.e.11] [359.3.e.5] [359.3.e.12]

Overzealous Fan's 'when I defend' trigger is placed on the chain as a triggered ability when it defends. [359.3.e.3] Players (including the attacking player) may add Reactions to the chain after the trigger is placed but before it resolves. The decision to perform the optional instruction 'you may kill me to move an attacking unit to its base' is made when that instruction executes as the triggered ability resolves — you do not have to kill Overzealous Fan immediately when the trigger is put on the chain. [355.10.c.1] At resolution, if you choose to kill Overzealous Fan the game attempts to execute the instruction(s). If parts are illegal at resolution (for example the chosen attacking unit is no longer a legal target), those parts are ignored while the rest execute if possible. [359.3.e.11] [359.3.e.5] If Overzealous Fan is no longer a legal object to be killed (for example it changed zones), you cannot perform the kill and the instruction that depends on it (moving an attacking unit) will be treated according to instruction legality rules. [359.3.e.12] [359.3.e.5]

If Overzealous Fan is removed from the board (moved to a non-board zone or otherwise becomes a different object) before the trigger resolves, it can no longer be killed as part of that instruction and the kill cannot be paid; any dependent parts of the instruction that require the kill will be ignored per [359.3.e.12]. If the attacking unit you would move is no longer a legal target at the time the ability resolves (for example it has moved to a non-board zone), the move portion is ignored even if you killed Overzealous Fan; follow the partial-instruction rules. [359.3.e.11] If the attacking unit was chosen as a target when the trigger was put on the chain and later only moved to base (not a non-board zone), whether it remains a valid target for the move depends on the targeting rules for that instruction; targets that cease to meet targeting requirements but later meet them again may still be considered chosen while on the chain. [359.3.e.3] If an opponent's Reaction attempts to change controller or status of Overzealous Fan while the triggered ability is on the chain, the information referenced by the triggered ability is taken from the trigger itself when it resolves; that can affect whether the instruction is legal at resolution. [359.3.f.4]

When Overzealous Fan defends, its triggered ability is placed on the chain. This puts the ability on the stack and gives both players an opportunity to play Reactions before that ability resolves. The line inside the ability — "you may kill me to move an attacking unit to its base" — is an instruction containing a cost (kill me) inside it. The Core Rules describe costs inside instructions as the cost portion of that instruction [355.10.c.1]. There is no rule text in the provided excerpts requiring that optional instructions be decided at the moment the trigger is placed on the chain; instead, instruction execution (including whether to perform a "may" instruction or to pay the cost embedded in it) occurs when the triggered ability resolves. Therefore you can wait while players add Reactions to the chain and then decide at resolution whether to kill Overzealous Fan to move an attacking unit.

If at resolution you choose to kill Overzealous Fan, the game will attempt to execute the instruction. If parts of the instruction are illegal at that time (for example the attacking unit you would move is no longer a legal target or Overzealous Fan is no longer present to be killed), then those parts are ignored or treated as null according to the instruction legality rules [359.3.e.11] [359.3.e.12] [359.3.e.5]. If the attacking unit was merely moved to a different valid location while the trigger was on the chain, the rules and FAQ explain how chosen targets can remain chosen even if they temporarily fail targeting requirements, which may affect whether the move can be performed when the trigger resolves [359.3.e.3].

In short: you may wait until the trigger resolves (after seeing any Reactions) to decide whether to use Overzealous Fan's optional kill-and-move instruction; make that choice at resolution, and then the game applies the instruction subject to the usual legality and partial-execution rules. [355.10.c.1] [359.3.e.3] [359.3.e.11] [359.3.e.5] [359.3.e.12]