Mystra, Lady of the Weave

Shroud, ethereal (This creature cannot be sacrificed by other cards or abilities.)
Whenever a player casts an instant or sorcery spell with converted mana cost {10} or higher, sacrifice Mystra, Lady of the Weave
The Spelllplague - When Mystra, dies, exile her. Until the beginning of your next end step, you may pay life instead of paying the mana costs of instants and sorcery spells you cast.