Does Thousand Tailed Watcher affect hidden units when they are revealed during showdown?

Ruling: No, Thousand Tailed Watcher does not affect hidden units that are revealed during showdown. It is not a persistent effect - it gives enemy units -3 might when played, and hidden cards are not units at that time.

Nuances:

• If an opponent responded to the Watcher's trigger by playing their hidden unit before the trigger resolved, that unit would get -3 might (though there's no strategic reason to do this)