Undercity Plunder 
Sorcery
Target opponent discards a card. Then they may discard an additional card. If they don't, conjure a duplicate of a random card from their library into your hand. It perpetually gains "You may spend mana as though it were mana of any color to cast this spell."
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyNOT LEGALvintageNOT LEGALcommanderNOT LEGAL