Can I react to Watcher ability?

Yes, you can react to the Thousand-Tailed Watcher's "When you play me" ability.

According to RiftJudge FAQ #93 and FAQ #5127, the Thousand-Tailed Watcher has a "When you play me" triggered ability that uses the chain, and players may respond to it using [Reaction] cards.

How the timing works:

1. Playing the Unit: When you play Thousand-Tailed Watcher, it enters the board. This does not create a chain and cannot be reacted to (per Rule 3722/7617).
2. Triggered Ability: Once the Watcher is on the board, its "When you play me" ability triggers and is placed on the chain.
3. Reacting: Because the ability is now on the chain, players can play [Reaction] cards.
   • Per Rule 406.4, other players have an opportunity to play Reactions before the resolution of spells and abilities on the chain.
   • Following the Showdown Timing Flowchart, if this occurs during a Showdown, the chain resolves in reverse order (LIFO - Last In, First Out).

Important Notes:

• No Actions: You can only use [Reaction] cards to respond to this trigger; you cannot play [Action] cards.
• Effective Timing: As noted in RiftJudge FAQ #2236, if you play a unit as a reaction to the Watcher's trigger, your new unit will be on the field when the chain resolves, meaning it will be affected by the Watcher's ability. If you play the unit after the trigger has already finished resolving, it will not be affected.