If Ezreal dashing attacks a battlefield controlled by Nine-Tailed Fox, does his ability go off before the might reduction?

No, Ezreal, Dashing's ability does not go off before the Might reduction.

According to the rules for combat-initiated showdowns and the specific interaction between these two cards, the Might reduction from Nine-Tailed Fox will resolve first.

The Sequence

1. Combat Begins: Ezreal, Dashing moves to the battlefield and is designated as the Attacker.
2. Triggers go on the Chain:
   • Ezreal's "When I attack" trigger is placed on the chain first (as the Active Player).
   • Nine-Tailed Fox's "When an enemy unit attacks" trigger is placed on the chain second (as the Defender/Legend controller).
3. Resolution (LIFO - Last-In, First-Out):
   • Nine-Tailed Fox's ability resolves first: The attacking unit (Ezreal) receives -1 Might.
   • Ezreal's ability resolves second: It checks Ezreal's current Might (which is now reduced) and deals that amount of damage to the target.

Why this happens

• Trigger Order: In a combat-initiated showdown, the active player (attacker) adds their triggered abilities to the chain first, followed by the defending player.
• LIFO Resolution: Because the chain resolves in Last-In, First-Out order, the last ability added (Nine-Tailed Fox) resolves before the first ability added (Ezreal).
• Dynamic Might: As established in RiftJudge FAQ #9638, there is no mechanism that "locks in" a unit's Might when an ability is added to the chain. The ability checks the unit's Might value only at the moment the ability actually resolves.

Note: This ruling is based on the standard combat timing flow and the interaction of triggered abilities on the initial chain.