When does Mask of Foresight's effect trigger, and what happens if you play a unit (like Teemo) from hidden after the trigger but before it resolves?

Ruling: Mask of Foresight triggers when a unit is designated as attacker/defender and is alone on a battlefield. Once triggered, the effect does not fizzle if another unit is played afterward - the original unit still gets the +1 buff.

Sequence:

• Unit is designated as attacker/defender while alone
• Mask of Foresight triggers and enters the chain
• Even if another unit (like Teemo) is played from hidden before the trigger resolves, the buff still applies
• The trigger does not check the "alone" condition again upon resolution

Nuances:

• This is different from passive effects (like Yi's) because it's a trigger that creates a pump buff lasting the entire turn
• The effect can trigger multiple times per turn if you engage in multiple combats
• The trigger does not undo itself once it has occurred