If I have two units at a battlefield with Mask of Foresight equipped, and the opponent attacks, can I use a reaction to move one unit away so the remaining unit gets the +1 might from Mask's 'when defends alone' trigger?

Ruling: No, Mask of Foresight will not trigger in this scenario. Attack/defend triggers only fire when a unit first gains the attacker/defender designation during a combat. Since both units were already designated as defenders when the showdown began, moving one away does not cause the remaining unit to be re-designated as a defender, so Mask's condition is never met.

Sequence:

• Opponent declares attack on battlefield with two defending units
• Both units gain defender designation simultaneously
• Mask checks conditions but sees two units, so does not trigger
• Moving one unit away removes its defender designation but does not re-apply defender designation to the remaining unit
• Mask cannot trigger because the remaining unit already had defender designation and is not gaining it "for the first time"

Nuances:

• If you move both units away and then move a single unit back in (e.g., using Ride the Wind), that unit would gain defender designation for the first time while alone, triggering Mask
• Mask can trigger multiple times in the same turn across different combats, but only once per unit per combat
• The trigger condition must be met at the moment the unit gains its attacker/defender designation, not checked continuously throughout combat