If I use Possession on a Watcher that was played this turn, whose units get -3 Might?

Ruling: When you take control of a Watcher with Possession, your units (the new controller's units) will get -3 Might to a minimum of 1 Might.

Sequence:

• Watcher is played and its trigger is created
• The trigger locks in what "enemy" units are at the time it's created (based on original owner)
• Possession is played, taking control of the Watcher
• The trigger resolves, affecting what were "enemy" units when the trigger was created (now your units)

Nuances:

• Taking control of a unit is different from playing it
• Control of the game object that created a trigger only matters when the trigger is being created, not when it resolves
• Once a trigger goes on the chain, its ownership is already determined