If you do, Identity Thief becomes a copy of that creature until end of turn
Well, with Altar of Dementia out, you can take out another player by eliminating their library altogether, giving them a very narrow window to do anything by the time their next draw step comes around

Foil-Only Future Sight Frame Cards 0243_MTGMB2_Fut_Foil: Foil 0244_MTGMB2_Fut_Foil: Urza, Lord High Artificer 0245_MTGMB2_Fut_Foil: Gix, Yawgmoth Praetor 0246_MTGMB2_Fut_Foil: Dragon's Approach 0247_MTGMB2_Fut_Foil: Squee, Goblin Nabob 0248_MTGMB2_Fut_Foil: Multani, Yavimaya's Avatar 0249_MTGMB2_Fut_Foil: Titania, Protector of Argoth 0250_MTGMB2_Fut_Foil: Captain Sisay 0251_MTGMB2_Fut_Foil: Gerrard, Weatherlight Hero 0252_MTGMB2_Fut_Foil: Mirri, Weatherlight Duelist 0253_MTGMB2_Fut_Foil: Xantcha, Sleeper Agent 0254_MTGMB2_Fut_Foil: Arcane Signet 0255_MTGMB2_Fut_Foil: Karn, Legacy Reforged 0256_MTGMB2_Fut_Foil: Command Tower 0257_MTGMB2_Fut_Foil: Reflecting Pool Playtest Cards Return First seen in Mystery Booster , playtest cards resemble the internal prototypes used by Wizards of the Coast when creating a Magic set
Is it a 28-day shout that ends in 337 days of silence
Anything, really, because they are barely known