The Battle Moon is a small satellite that orbits Finn's head and shields him from attacks. He gets it in the episode "Dungeon Train" as loot from one of the bosses on the Dungeon Train. When one of the Hair Apes shoots its electricity at Finn, the battle moon shoots a laser at the bolt, blocking the attack. In the future crystal, Finn still has the battle moon, but it orbits a miniature earth, which in turn orbits Finn's head. When Finn gets up to return the crystal to Jake, the battle moon drops to the floor, suggesting that Finn can control its orbit.
It is gray and is about the size of a golf ball. Several small craters dot its surface, and a small beacon stands at its north pole. The beacon shoots a yellow laser when defending.