Dandân, also known as Forgetful Fish, is an unofficial two-player Magic micro-format created by Nick Floyd. Its traditionally played from a single shared mono-blue deck and graveyard. The most familiar versions use an 80-card deck built around Dandân itself and spells that manipulate shared resources.
Cards such as Vision Charm can manipulate the Islands that Dandân needs to survive and attack, while Memory Lapse and Predict create particularly nasty mind games when both players are fighting over the same library.