Dack Fayden emblem

[sithantic]

His emblem states that "Whenever you cast a spell that targets one or more permanents, gain control of those permanents." Now, if I cast a spell like {Mutant's Prey}, the second target has to be a creature my opponent control's. So, I target their creature, and the emblem triggers. I then gain control of the creature. When Prey goes to resolve, will it actually be countered since the target no longer belongs to my opponent?

[particle]

They will not fight. One of the targets is illegal upon resolution.

[Anoobass]

But the opponents creature is a legal target when the spell is cast, does it really need to be legal for the spell to resolve?

[MisterJH]

^ yes

[DYMongoose]

But the opponents creature is a legal target when the spell is cast, does it really need to be legal for the spell to resolve?

Consider:
{Shock} -> {llanowar Elves}
Response: {Sheltering Word} -> {Llanowar Elves}
[pass], [pass]
Sheltering Word resolves: elves have hexproof; Sheltering Word's controller gains 1 life.
[pass], [pass]
Shock is countered because its target is no longer legal.