Does Thousand-Tailed Watcher's ability start a chain and can it be responded to with reactions?

Question

Accepted Answer

If Thousand-Tailed Watcher has a Triggered Ability, that ability is placed on the chain when its trigger condition is fulfilled and can be responded to with Reactions. The controller may need to make choices/targets when placing it on the chain, and those choices must be valid; Reactions may be played while the Triggered Ability is pending on the chain. [383.3.c] [355.8] [813.1.c.1]

Yes — a Triggered Ability of Thousand-Tailed Watcher will be put on the chain when its trigger condition is fulfilled, and players may respond to that chain item with Reactions. The ability’s controller must finalize any required choices/targets when placing it on the chain; once it is on the chain, other players can use Reactions to respond. [383.3.c] [355.8] [813.1.c.1]

The trigger condition for Thousand-Tailed Watcher becomes fulfilled; the ability becomes eligible to be placed on the chain. [383.3.c] If the Triggered Ability contains a leading 'you may' that governs whether it goes on the chain, the controller chooses whether to place it on the chain. [383.3.c] When placing the Triggered Ability on the chain, the controller finalizes any required choices and targets; those choices must be valid at that time. [355.8] Once the Triggered Ability is on the chain as a pending item, players may respond to it with Actions and Reactions, because Reactions are permitted timings that allow play during Closed States and thus can be used to respond to chain items. [813.1.c.1] [383.3.c] When the chain resolves, any targets that became illegal after finalization are ignored for the relevant instructions; other parts of the ability still happen as applicable. [359.3.e.5]

If the Watcher’s ability includes an additional condition that must be true to be placed on the chain (for example 'When X happens, if I'm at a battlefield'), that conditional is checked before placing the triggered ability on the chain — if it’s false, the ability isn’t placed. [383.3.e] If the Triggered Ability contains costs in its instructions (e.g., 'Recycle me to...'), those costs become the base cost of the Triggered Ability and must be paid to finalize it on the chain. [383.3.b.1] If the ability targets something and its target becomes illegal between finalization and resolution, that portion of the instruction is ignored while the rest resolves. [355.8] [359.3.e.5] See the Arcane Shift / Thousand-Tailed Watcher example in the clarifications for a similar sequencing principle. [FAQ:unleashed#16]

Triggered Abilities are put on the chain when their trigger condition is fulfilled; they can be placed on the chain during Closed or Open States on any player's turn, so they behave like other chain items and can be responded to while pending. [383.3.c] When the ability is being placed on the chain, its controller must make any required choices and select any targets; those choices must be valid at that time or the ability cannot be finalized in that form. [355.8] Reactions are a permission that lets a card or ability be played during Closed States on any player's turn (they are functionally short for allowing play as a response), so players can use Reactions to respond to a Triggered Ability that is on the chain. [813.1.c.1] If the Triggered Ability includes costs noted in its instructions, those costs are treated as the base cost of the Triggered Ability and must be paid to finalize it on the chain. [383.3.b.1] Finally, if a target chosen during finalization becomes illegal before the ability resolves, the targeted part of the instruction is ignored while other instructions still resolve as applicable. [359.3.e.5] Because you did not provide the actual text of Thousand-Tailed Watcher, I treated it as a standard Triggered Ability for this ruling; if its text is a Replacement Effect or is otherwise worded to avoid being a triggered ability, that would change the sequencing and whether it becomes a chain item. [FAQ:unleashed#2]