When an opponent card attacks a battlefield, playing a Tideturner from Hidden and switching in an Overzealous Fan, can I still use the fan's ability?

Yes, you can use Overzealous Fan's ability after swapping it into the battlefield with Tideturner during a showdown.

According to RiftJudge FAQ 9047 and FAQ 9411, swapping Overzealous Fan to the contested battlefield keeps it on the board, which satisfies the requirement for paying the "kill me" cost of its ability.

Sequence of Events

1. Showdown Begins: An opponent's unit attacks a battlefield. If Overzealous Fan is moved into that combat (or is already there), it gains the "Defender" designation.
2. Reaction: You play Tideturner from Hidden as a reaction. This starts a chain.
3. Tideturner Resolves: Tideturner enters the battlefield and its "When you play me" ability triggers. You swap Tideturner with Overzealous Fan.
4. Fan's Ability: Because the Fan is now at the contested battlefield, it gains the "Defender" designation (if it didn't have it already). Its "When I defend" ability triggers and is placed on the chain.
5. Resolution: When the chain reaches the Fan's ability, you may choose to kill the Fan to move the attacking unit to its base. Because the Fan is still on the battlefield, it remains a valid target for its own "kill me" cost.

Important Notes

• No Whiff: As long as the Fan remains on the battlefield (even if it is at a different location than where it started), the ability does not "whiff" and can be resolved successfully.
• Targeting: The target of the Fan's ability (the attacking unit) was locked when the ability was placed on the chain. Moving the Fan does not invalidate this target.
• Defender Designation: Per RiftJudge FAQ 9174, if you swap a unit into the defender position during a showdown, it gains the "Defender" designation during the cleanup phase following the resolution of the swap, which then triggers the "When I defend" ability.