Does Thousand-Tailed Watcher's -3 might effect persist as a perpetual debuff throughout the turn, or does it only apply once when it resolves?

Ruling: Thousand-Tailed Watcher's -3 might effect is not perpetual. It applies once when the ability resolves, reducing all face-up units to a minimum of 1 might, then the effect is complete. Any subsequent might changes start from the new value.

Sequence:

• Watcher's ability resolves, giving all face-up units -3 might (minimum 1)
• The effect completes and does not linger
• Any buffs or debuffs applied afterward modify the unit's current might normally
• Units that enter the battlefield or are revealed after Watcher's ability resolves are not affected by it

Nuances:

• Hidden units are not affected by the -3 when Watcher's ability resolves, since they are not face-up on the battlefield at that time
• If a unit is revealed after Watcher's ability has resolved, it does not receive the -3 debuff