Jake's sword first appears in the episode "Tree Trunks." Jake waves it around while shouting "Swords!" with Finn.

It reappears in "Conquest of Cuteness" when Finn and Jake catch two Cuties trying to steal it.

Appearance

The sword has an unusual appearance, the hilt being longer than the actual blade. It has a skull for a pommel and an eye on the guard. Half of the blade is serrated on one edge. Jake most likely is not proficient with this sword, as he is seldom shown using it.

Trivia

In real life, there is a toy version of Jake's sword.

There is also a real life version of the sword itself. The video can be found here.

Jake has never actually been shown using this sword for battle.