Can you react to Thousand Tailed Watcher's ability that gives -3 might to all units?

Ruling: Yes, you can react to "When you play me" effects like Thousand Tailed Watcher's ability.

Nuances:

• You can react to most things except gears and units, as they resolve instantly
• When reacting, you can only respond with a reaction (not an action)
• After the ability resolves, you don't have a chance to play an action
• Actions can only be played on your turn and when starting chains in showdowns
• Spells without an action or reaction keyword have the same timing restrictions as units (only on your turn, not in showdowns)