Kylox, Visionary Inventor - Ravnica: Murders at Karlov Manor

Menace, ward {2}, haste
Whenever Kylox, Visionary Inventor attacks, sacrifice any number of other creatures, then exile the top X cards of your library, where X is their total power. You may cast any number of instant and/or sorcery spells from among the exiled cards without paying their mana costs.