Does Ahri Nine-Tailed Fox's ability give -1 Might to all attacking units or just one unit?

Ruling: Ahri Nine-Tailed Fox's ability triggers separately for each enemy unit that attacks a battlefield she controls, giving each attacking unit -1 Might (to a minimum of 1 Might).

Sequence:

• When an enemy unit attacks a battlefield Ahri controls, her triggered ability activates
• The ability triggers separately for each attacking unit
• Each trigger gives -1 Might to one unit
• The Ahri player must announce the trigger when it happens (when combat damage occurs is acceptable timing)

Nuances:

• This is a triggered ability (indicated by "when"), not a passive ability, so it must be announced
• If the trigger is not announced before it would affect the game in a visible fashion, it can be considered a missed trigger
• The ability can apply more than once per turn since it lasts until end of turn
• The wording "an enemy unit" means each individual unit triggers the ability separately, not that only one unit total is affected