When is Overzealous Fan activated? After or before actions is passed, do you need to pay the deflect cost? Can I counter the effect with Not So Fast?

Overzealous Fan's ability triggers the first time it gains the Defender designation during a combat showdown — that's its Defend Trigger (Rule 383.4.f).

If the Fan is already at the battlefield when combat opens, it gains Defender at combat start and triggers then. If it enters mid-combat (e.g., via Moonfall or a swap effect), it gains Defender at the next Cleanup and triggers at that point — but only once per combat (383.4.f.2.a). Because the ability says "you may kill me," using it is optional, and the kill is paid upfront as a cost to put the trigger on the chain — so countering the ability doesn't save the Fan (FAQ 8702, Rule 383.3.b.1).

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