Narset, Enlightenment Seeker

Hexproof, Prowess
Whenever a player casts his/her first instant or sorcery each turn, you may pay {x}, where X is the amount of mana he/she spent to cast the spell. If you, exile it after it resolves.
At the beginning of your upkeep, you may cast spells exiled with Narset without paying their mana costs. If 3 or more spells have been cast this way, exile Narset, Enlightenment Seeker and return her onto the battlefield transformed.