When Mirror-Mad Phantasm enters the battlefield, Unstable Shapeshifter triggers, causing Unstable Shapeshifter to become a copy of Mirror-Mad Phantasm until end of turn.
Activate Unstable Shapeshifter by paying ({1} magic symbol) ({U} magic symbol) , shuffling Unstable Shapeshifter into your library and milling your entire library due to it not having a card named Mirror-Mad Phantasm in it.