The following contains plot details/spoilers, read at your own discretion:
Magical Girl Raising Project
After La Pucelle's death at the hands of Cranberry during her selection test, Fav send a message to the Land of Magic. He blamed the death on an accident and demanded that Cranberry be given magical items under the pretext so that she could better protect the remaining participants.
The Land of Magic send five items, including a weapon that came in the form of a halberd. Fav and Cranberry would, however, not use them to protect the other participants. Instead they wanted to use them to make the fighting even bloodier. Fav set up an online shop where participants could buy the items in return for their life span.
Under Swim Swim's instructions, Tama purchased the halberd. However, since the others magic was more suited for it, Swim Swim and Tama exchanged their items. Being allowed to name the halberd, Swim Swim decided to name it "Ruler", after her deceased mentor. Throughout the selection test, Swim Swim used Ruler to kill Weiss Winterprison, Top Speed, Hardgore Alice and Tama, as well as handicap Ripple.
After Magical Girl Raising Project
Following Swim Swim's death, Ripple took possession of Ruler. It was eventually pased on to Snow White, whom would use it during battles against Rogue Magical Girls. Because of Snow White's fame as the Magical Girl Hunter, Ruler would in turn also become a very famous weapon.
The following might reveal some plot details/spoilers, read at your own discretion:
Ruler has the ability of being completely unbreakable and indestructible. It is also very powerful, being able to easily damage the body of a Magical Girl.