Mjolnirs are two handed member's only weapons. There are three different mjolnirs, each affiliated with a particular god: Guthix, Saradomin, or Zamorak. The mjolnirs are tipped with symbols of their corresponding gods, with a tear-drop symbol for Guthix, a four pointed star for Saradomin, and a pair of horns for Zamorak.
There are no requirements for wielding mjolnirs, however their stats are unimpressive.
Due to an update, you can re-obtain a new key after completing the Making History quest, allowing you get the mjolnirs again.