Epictetus 
The Enchiridion 

The Enchiridion (which translates to 'The Handbook’ or 'The Manual’) was an ancient book with codes of conduct, guidelines, and other helpful information for heroes. The book had great power which allowed it to tear open worm holes between dimensions in the multiverse. It was broken into pieces after turning to stone by Finn in 'The Lich’ in order to stop the Lich from gaining access to the multiverse, but, as it turned out, by doing this Finn made the portal to the multiverse. (See 'The Lich’ for more details.) It was revealed in 'Winter Light’ that while Finn and Jake were in the Farmworld Dimension during the events of 'Crossover, ’ Finn brought the Enchiridion from that timeline back with him.