3. The Mushroom War
What I'm referring to here is the war that ended the world as we know it, and eventually spawned the magical world known as Ooo and its creatures. We know thanks too the episode "Finn the Human" that this event occurred around 1000 years ago, and that it caused the near extinction of human beings (Finn being the only exception). We are also shown that the fallout from this blast somehow created Finn's arch nemesis, The Lich King, but our knowledge of this vital event ends there. Who was involved in this war? How long did it go for? What were the casualties? I feel these questions may never get answered due to the shows nature. However such an unspoken, world changing event sparks a large amount of curiosity within me.