Does Thousand-Tail Watcher -3 might effect still go off if its star-crossed in response to the trigger?
Yes, the Thousand-Tailed Watcher's -3 Might effect will still resolve even if you use Star-Crossed in response to the trigger.
According to RiftJudge FAQ #9750, removing the source of an ability from the battlefield does not prevent the ability from resolving once it is on the chain.
Sequence of Events
- Playing the Unit: Your opponent plays Thousand-Tailed Watcher. It enters the battlefield. This action does not use the chain.
- Triggered Ability: The Watcher’s "When you play me" ability is placed on the chain. The game enters a Closed State.
- Playing the Reaction: You play Star-Crossed in response. You target your friendly unit and the Thousand-Tailed Watcher. Star-Crossed is placed on top of the chain.
- Chain State (Top to Bottom):
- [Top] Star-Crossed
- [Bottom] Thousand-Tailed Watcher's Ability
- Resolution (LIFO - Last-In, First-Out):
- Star-Crossed resolves first: It returns your friendly unit and the Thousand-Tailed Watcher to their owners' hands.
- Watcher's ability resolves second: Even though the Thousand-Tailed Watcher is no longer on the board, its ability was already on the chain. As confirmed by RiftJudge FAQ #9750, abilities do not "fizzle" just because the source unit leaves the battlefield. The effect will proceed to resolve, applying -3 Might to all enemy units currently on the battlefield.
As noted in RiftJudge FAQ #10506, because the Watcher's effect is a blanket effect that programmatically selects all enemy units rather than targeting them individually, it cannot be countered by removing the source or by using protection effects that require targeting.
Note: I could not fully verify this ruling against the retrieved rules, as the cited Rule 359.3.e.5 specifically addresses illegal targets for spells rather than the resolution of abilities from departed sources.