As long as there are five or more charge counters on Mirror of Wishes, you may cast cards you own from exile.

: Each player exiles the top card of his or her library. Put a charge counter on Mirror of Wishes.

,

: Exile target creature. Put a charge counter on Mirror of Wishes. This ability costs

less to activate for each charge counter on Mirror of Wishes.
The key item of Mirror Magic. Powerful, innovative, highly plot-relevant, a slam-dunk mythic.
Works like a mix between Treasure Map and Altar of Shadows: you start off by just exiling a few cards and charging up a bit (proliferate helps this stage go faster). Then you either get enough mana to start zapping things, and it only gets cheaper from there; or you get up to 5 charge and this is effectively an Urza's Blueprints. Interacts with the proliferate and mill themes in the set as well.
Inexorably powerful, as suits a mythic; but doesn't do much on the actual turn you cast it unless you have 11 mana.