If Darius (with Hold) has 3 damage dealt to it by Hextech Ray, then Thousand Tail Watcher reduces its Might by 3, does Darius die?

Ruling: Yes, Darius will die. The game checks if damage marked on a unit is greater than or equal to its current Might, so when Watcher's ability resolves and reduces Darius to 2 Might while it has 3 damage marked, Darius dies.

Sequence:

• Hextech Ray resolves, dealing 3 damage to Darius
• Thousand Tail Watcher is played, reducing Darius's Might by 3
• Game checks damage >= Might (3 damage >= 2 Might)
• Darius dies

Nuances:

• The order of damage being dealt first does not prevent the Might reduction from causing death; the game continuously checks if damage marked meets or exceeds current Might