Does Thousand-Tailed Watcher's ability apply to Shen if Shen is played as a reaction after Thousand-Tailed Watcher triggers on the same turn?

Ruling: If Shen is played as a reaction to Thousand-Tailed Watcher's trigger (or to anything that reacted to it), Shen will be affected by the ability because the chain resolves backwards and Shen hits the field before the trigger resolves. If Shen is played later in the turn after the trigger has already resolved, he will not be affected.

Sequence:

• Thousand-Tailed Watcher's trigger goes on the chain
• Shen is played as a reaction to the trigger (or to another reaction)
• Chain resolves backwards: Shen enters the field first
• Thousand-Tailed Watcher's trigger resolves, affecting all units currently present including Shen

Nuances:

• Thousand-Tailed Watcher's trigger affects every unit present at resolution, not just units that were on the field when it triggered
• The same principle applies to other spells or abilities that affect all units (they affect the "current" all, such as Grand Stratagem)
• If Shen is played after the trigger has already resolved (later in the turn), he will not be affected by the -3 Might