If Mask of Foresight is active and a unit is designated as a defender alongside other units, then during the initial chain those other units are moved away leaving only one defender, does Mask trigger or give the remaining unit +1 might?

Ruling: Mask of Foresight does not trigger and does not give the remaining unit +1 might. The trigger condition for Mask ("when I attack/defend alone") must be met at the moment units are designated as attackers/defenders, which only happens during the initial chain of a showdown.

Sequence:

• Units are designated as attackers/defenders (this is when "when I attack/defend" triggers check)
• If a unit is alone at this moment, Mask triggers and goes on the stack
• If Mask triggered, it will give +1 might to "that unit" (the specific unit that triggered it) when it resolves, even if that unit is no longer alone
• If a unit was not alone when designated, Mask does not trigger at all
• Moving units in or out of the battlefield after designation does not create new Mask triggers

Nuances:

• Mask does not target, so it doesn't check conditions again on resolution - it will resolve and give might to the unit that triggered it even if the battlefield state has changed
• The might goes specifically to "that unit" (the one that triggered Mask), not to whichever unit happens to be alone when it resolves
• If the unit that triggered Mask is removed before Mask resolves, the effect fizzles and no other unit gets the might
• A hidden unit revealed after designation was not alone when it became a defender, so it cannot trigger Mask or receive might from an existing Mask trigger