Can Fiora Grand Duelist's effect be used to channel a rune exhausted when one of your units becomes mighty during your opponent's turn?

Ruling: Yes, Fiora's effect can be used during your opponent's turn when one of your units becomes mighty. It is a trigger, not an activated ability, so it will trigger regardless of whose turn it is.

Nuances:

• The effect does not need to state "reaction" to trigger during an opponent's turn
• Triggers work differently from activated abilities in terms of timing