Zannathorn, Bringer of Justice

Zannathorn, Bringer of Justice is only and automatically cast when Blade of Zannathorn and Hilt of Zannathorn are equipped to the same creature.
Equipped creature gets either double toughness or double power, your choice. It also becomes legendary, and gets "when this creature dies, you may pay {x}. If so, return it to the battlefield with a +{x}/+{x} counter on it.