Mirror-Mad Phantasm in hand or on the battlefield or in the command zone or in exile or in your graveyard.
Mercurial Pretender is a copy of Mirror-Mad Phantasm.
({1} magic symbol) ({U} magic symbol) available.
Steps
Activate Mercurial Pretender by paying ({1} magic symbol) ({U} magic symbol) , shuffling Mercurial Pretender into your library and milling your entire library due to it not having a card named Mirror-Mad Phantasm in it.