For Challenge # 038.

Clearly had to have this cost and name.

Almost certainly doesn't work as written; but the intent is hopefully clear. Almost always just works as a Counterspell; but just sometimes works a lot better.

This seems pretty fair. (Eg. comparable to Commandeer)

For the wording, how about "Exile target spell. You may cast it from exile until end of turn."

What, like a souped up Shunt? Yeah, I guess that is fair as long as you don't get to keep a permanent after the fact. That would be Spelljack, minus the timing issues. If you just want to turn Fire from Fire // Ice into Ice however, and change the targets, then should get you there (though, not without people flipping out about the ability being impossible).

Humm; as I worded it, you would get to keep the permanent I think - I wanted spells that say "You" to affect the new you, but as you say, that's a problem.

Exile the spell, then allow you to cast it probably works most sanely rules wise; though still allows keeping the permanent.

Yeah, I think this just wants to be Spelljack or Commandeer.

Make it a Spelljack for Instants and Sorceries only. That was the original intention, wasn't it?

Not really; the original intention was "Counter target spell, unless you pay its mana cost too; in which case, go for it, cast that spell."