Undercity Plunder

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."

From Magic: The Gathering.

Open the interactive version on https://mythscribe.cloud/app/tcg/cards/cms6ogbkfbptdpb1hl7461yaz.