Mjolnirs are two-handed weapons that can be dug up after the Making History quest by using the enchanted key. There are three mjolnirs to be found: the Saradomin mjolnir, Zamorak mjolnir and Guthix mjolnir. Despite being affiliated with the gods, they do not provide protection from their related factions in the God Wars Dungeon. In addition, they have stats roughly equivalent to that of an Iron 2h sword. These reasons make the Mjolnirs seem entirely useless and probably designed for cosmetic purposes.
A player can only find each mjolnir once, as the enchanted key disintegrates when all of its treasure has been found. If the player loses or sells a mjolnir, the only way to get a new one is by trading with another player.