Finn Sword

The Finn Sword is created in "Is That You?" after Finn sacrifices an alternate version of himself to save Prismo. After visiting a much more elaborate version of Prismo's Time room, created as a "Plan B," Finn makes a choice to save Prismo by creating a paradox that vaporizes his dream world duplicate. Instead of turning to dust, Finn's body turns into a sword, dubbed the Finn Sword by Prismo. The Finn Sword first appears in "Breezy" when Finn awakens to Breezy's final song. However, instead of seeing Breezy in her Queen form, he has a vision of Princess Bubblegum holding the Finn Sword.

Abilities
The sword is first used in "Dentist" and is shown to have Finn's dream world duplicate trapped inside of it and able to speak directly to Finn from the blue sphere it is encased in. Also, while Finn is using this sword in battle, his physical attributes are double what they are without it. This is more than likely linked to the dream world duplicate inside it.