Horobi's Whisper | Magic: The Gathering
Formats Horobi's Whisper is Legal in
Official Oracle Text for Horobi's Whisper
Splice onto Arcane—Exile four cards from your graveyard. (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)
Official Rulings for Horobi's Whisper
6/7/2013 : You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell.
6/7/2013 : A card with a splice ability can't be spliced onto itself because the spell is on the stack (and not in your hand) when you reveal the cards you want to splice onto it.
6/7/2013 : You choose all targets for the spell after revealing cards you want to splice, including any targets required by the text of any of those cards. You may choose a different target for each instance of the word "target" on the resulting spell.
6/7/2013 : If all of the spell's targets are illegal when the spell tries to resolve, it will be countered and none of its effects will happen.