When does Overzealous Fan trigger? And can it trigger after both players have started a chain or if things go south?

Overzealous Fan's ability is a Defend Trigger. According to Rule 383.4.f.3 and the Unleashed update, it triggers the first time the unit gains the "Defender" designation during a combat showdown.

When does it trigger?

It triggers when the unit gains the Defender designation. If the unit enters combat "late" (e.g., via a swap effect like Tideturner), it will trigger during the cleanup phase immediately after it gains the Defender designation (FAQ 9174).

The "Kill Me" Cost and "May" Choice

Based on the current rules (specifically Rule 383.3.a, 383.3.b, and FAQ 8702), the "may" and the "kill me" cost function as follows:

1. Trigger Condition Met: When the Fan gains the Defender designation, the ability triggers.
2. The "May" Choice: Because the ability says "you may," the choice to place it on the chain is optional (Rule 383.3.a).
3. Paying the Cost: If you choose to use the ability, you must pay the cost—killing the Overzealous Fan—upfront to place the ability on the chain (Rule 383.3.b.1, FAQ 8702).
4. Result:
   • If you choose not to use it: You do not kill the Fan, and the ability never goes on the chain.
   • If you choose to use it: You kill the Fan immediately as a cost. The ability is then placed on the chain as a pending item.

Can it trigger after chains have started or "if things go south"?

• After chains have started: Yes. If the Fan gains the Defender designation mid-showdown (e.g., after an initial chain has already resolved), the ability will trigger at that time.
• "If things go south" (Countering): If you pay the cost (killing the Fan) to put the ability on the chain, and an opponent plays a card like Not So Fast to counter the ability, the Fan remains dead. Because the kill was a cost paid to place the ability on the chain, countering the ability does not "undo" the cost (Rule 425.1.c).

Summary of Interaction

• Targeting: You must choose the target attacking unit when you place the ability on the chain (FAQ 270).
• Priority: Once the ability is on the chain, players can respond with reactions (FAQ 270).
• Showdown Continuity: The showdown does not end if the Fan is killed or if the unit is moved; players can continue to play actions and reactions until both players pass consecutively (FAQ 8798).

Note: FAQ 8702 serves as the authoritative source for the "cost-upfront" mechanic and resolves previous contradictions regarding this interaction.