Refraction Trap doesn’t target the source of the damage it prevents. You choose that source as Refraction Trap resolves. If you cast Refraction Trap for {W}, the source you choose doesn’t have to be the red instant or sorcery spell an opponent cast.
If the chosen source would deal damage, Refraction Trap prevents 3 of that source’s damage and the source deals its excess damage (if any) at the same time. Immediately afterward, as part of that same prevention effect, Refraction Trap deals its damage. This happens before state-based actions are checked, and before the spell or ability that caused damage to be dealt resumes its resolution.
If the chosen source would simultaneously deal damage to multiple permanents you control, or would simultaneously deal damage to you and at least one permanent you control, you choose which of that damage to prevent. For example, if the chosen source is Earthquake, you might choose to prevent the next 2 damage it would deal to you and the next 1 damage it would deal to a creature you control. You don’t decide until the point at which the source would deal its damage.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Zoltan Boros & Gabor Szikszai
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If Refraction Trap doesn’t prevent any damage (perhaps because a different prevention effect is applied to the damage the source would deal, or because the damage is unpreventable), Refraction Trap won’t deal any damage itself.
Refraction Trap’s only target is the permanent or player it may deal damage to. You choose that target as you cast Refraction Trap, not at the time it prevents damage.
Refraction Trap’s effect is not a redirection effect. If it prevents damage, Refraction Trap (not the chosen source) deals damage to the targeted permanent or player as part of that prevention effect. Refraction Trap is the source of the new damage, so the characteristics of the original source (such as its color, or whether it had lifelink or deathtouch) don’t affect this damage. The new damage is not combat damage, even if the prevented damage was.
Whether the targeted permanent or player is still a legal target is no longer checked after Refraction Trap resolves. For example, if a creature targeted by Refraction Trap gains shroud after Refraction Trap resolves but before it prevents damage, Refraction Trap will still prevent damage and still deal damage to that creature. If Refraction Trap can’t deal damage to the targeted permanent or player (because the creature is no longer on the battlefield, or is no longer a creature, or the player is no longer in the game, for example), it will still prevent damage. It just won’t deal any damage itself.
Refraction Trap can prevent damage that would be dealt to you, one or more creatures you control, and/or one or more planeswalkers you control.
If the targeted permanent or player is an illegal target by the time Refraction Trap resolves, the entire spell doesn’t resolve. No damage will be prevented.
