I control a
Vedalken Shackles and 6 islands. My opponent controls a
Timbermaw Larva and 6 forests. My opponent attacks me with the Larva, attack trigger on the stack. I respond by taking activating the Shackles targeting the Larva. When the trigger resolves, does it count the number of forests my opponent controls (since he controls the trigger) or the number I control (since I control the creature).
Your opponent still controls the trigger. "You" in the triggered ability refers to the controller of the triggered ability. So it counts the number of forests your opponent controls.
CompRules wrote:
109.5. The words “you” and “your” on an object refer to the object’s controller, its would-be controller (if a player is attempting to play, cast, or activate it), or its owner (if it has no controller). For a static ability, this is the current controller of the object it’s on. For an activated ability, this is the player who activated the ability. For a triggered ability, this is the controller of the object when the ability triggered, unless it’s a delayed triggered ability. To determine the controller of a delayed triggered ability, see rules 603.7d–f.
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If it does count his forests, do I retain control of the creature even though its power is now greater than the number of islandsI control?
Yes, you do retain control of it.
"Creature with power less than or equal to the number of Islands you control" is just a targeting restriction, so the power and the number of islands are only checked when the ability is activated and when it resolves. After that, what you have is a continuous effect that only cares about the Shackles being tapped.
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Thanks for your help!