From Bad to Worse Achievement in Adventure Time: The Secret of the Nameless Kingdom

Story related and cannot be missed.

To acquire the renewable light source, you need to first have acquired Grabby Hands from the first temple. After you have done so, follow the steps in “A day in the life of a bun.” Once Cinnamon Bun is out of the way, go two screens to the right to find Rattleballs and a tiny manticore flying around. 

Use Grabby Hands to put the manticore in an empty baggy. Next, you need to go to Magic Man and give him the manticore. Magic Man is located in a house one screen to the right from Stanley which is in the very bottom left corner of the map.