Adventure Time With Finn and Jake - Simon and Marcy (S05E11) HQ

Marceline decides to tell Finn and Jake about her and the Ice King's adventures 996 years ago following the Mushroom War. In her story, Marceline and Simon—the name of the Ice King before he was corrupted by his crown—traverse a ruined city to find chicken soup to cure a cold that Marceline has. Along the way, they are attacked by mutated creatures, forcing Simon to wear the ice crown and slowly lose his mind.