Can I use Fiora Grand Duelist mid Showdown (with Laurent Duelist)?

Yes, you can use the ability of Grand Duelist (the Legend) during a Showdown, provided the trigger condition is met.

Ruling and Interaction

Grand Duelist is a Legend, not a unit. According to Rule 170.7, Legends can have Triggered Abilities. When one of your units becomes Mighty (its Might changes from less than 5 to 5 or greater), the Legend's ability triggers.

Because this is a triggered ability, it uses the chain. Once triggered, it is placed on the chain and can be responded to by either player.

Timing and Resolution

1. Trigger: When a unit becomes Mighty during a Showdown, the Grand Duelist's ability is added to the chain.
2. Chain Resolution: Chain items resolve using LIFO (Last In, First Out), meaning the most recently added item on the chain resolves first.
3. Mid-Showdown Usage: You can trigger this ability mid-Showdown. If the Showdown is in a "Closed State" (a chain exists), you can add this trigger to the chain. If the chain is empty, you can start a new chain with this trigger.

Example Sequence

• Laurent Duelist is in combat.
• You play a card or effect that increases Laurent Duelist's Might to 5 or greater.
• The effect resolves, and Laurent Duelist "becomes Mighty."
• Grand Duelist's ability triggers and is placed on the chain.
• Players may now add reactions to the chain.
• The chain resolves in LIFO order: the most recently added reaction resolves first, followed by the Grand Duelist's ability (allowing you to exhaust the Legend to channel a rune).

Note: I could not fully verify this ruling against the retrieved rules.