Does Ruin Runner get -3 Might when Thousand Tail Watcher is played?

Yes, Ruin Runner will receive the -3 might reduction from Thousand-Tailed Watcher.

The reason for this interaction is that Thousand-Tailed Watcher's ability does not choose (target) specific units. It applies a global effect to all enemy units. Ruin Runner's ability states, "I can't be chosen by enemy spells and abilities," which only protects it from effects that require a player to specifically target it.

Since Thousand-Tailed Watcher affects the entire board without targeting, Ruin Runner is affected normally.

Key Details:

  • Targeting: Because the Watcher's ability doesn't use the word "choose" or "target" for the units it affects, it bypasses Ruin Runner's protection.
  • Minimum Might: Per the text of Thousand-Tailed Watcher, Ruin Runner's might will be reduced from 5 to 2 (it cannot be reduced below 1).
  • Timing: This reduction only applies to units already on the board when Thousand-Tailed Watcher's "When you play me" ability resolves.