Simon was a treasure hunter in the future, maybe around 50 years from now. I say around 50 years due to the advanced technology shown constantly around in the series. Having stumbling upon the Enchiridion (You can see he has it in I Remember You) his curiosity peaks when he finds a spell to open the Nightosphere. Having discovering the Ice Crown, he has the power to open the portal. He opens the portal, releasing evil upon the world.
Then the Mushroom War happens, and I don't know what sparked it. But I can say for certain that it happened very suddenly. You can still see people frozen in place watching Television in their homes in the episode Ocean of Fear. This most likely caused the radiation that caused the humanoid persons currently inhabiting Ooo. Although, I can't say for certain again. All we know for sure is that a HUGE part of the Earth was literally destroyed in the War as shown in the episode Five Short Graybles.
Having the Ice Crown, Simon survives. He has been under the control of the Ice Crown for some while when he stumbles upon Marcelline, a vampire child that was released from the Nightosphere. You can tell he has had the Crown for a while as his skin is a little blue and has a growing beard. He gets her toy Hambo, which she later cherishes as her most valued possession.
Something happens between Marcelline and Simon. Maybe his crowns makes him say something very hurtful and it drives Marcelline away. I know this because Princess Bubblegum has not shown any knowledge of the connection between Marcelline and the Ice King. As shown in the episode What Was Missing, Marcelline and Princess Bubblegum have a history of being good friends. The only way PB could not have known of Marcelline's connection to the Ice King is if Marcelline and Simon haven't seen each other in ages. This leads me to believe that she abandoned Simon as he slipped further into madness. Seeing this, I believe he wrote his final memoirs before finally being cast into the deepest part of the Ice Kings mind. Marcelline flees from the Ice King, but as she said, he keeps following her and moving into nearby places. He is most like subconsciously following her, not remembering of his past with her but is just drawn to her because of their old connection. All I know for sure is that he went crazy from loneliness and abandonment over the course of over a thousand years (He survives by the Ice Crown's magic) and it shows in his character.
Now, the Lich wants the Enchiridion and successfully acquires it as shown in episode In Your Footsteps. He is most likely, if my theory is correct, going to try and open the portal to the Nightosphere. I'm not talking about a small portal like in the episode It Came From the Nightosphere. I'm talking about a big enough portal that allows enough evil and demons to enter Ooo, to the point where the original inhabitants had no choice but to go nuclear. From previous episodes, it leads me to believe that the Lich feeds off of radiation. Having inhabited Princess Bubblegum in the episode Mortal Recoil, he orders Jake to make a drink with Plutonium, so I can conclude this to be at least partly true. So in the next episode The Lich, the lich might want to start another nuclear war.