Myrtenaster is Weiss Schnee's weapon, a Multi Action Dust Rapier. Myrtenaster is highly durable, slashing away at the Giant Armor with no visible wear on the blade.
Myrtenaster's most prominent ability lies in the revolver-like mechanism held within the hilt, which contains various colors of Dust, activated by a trigger on the underside of one of the prongs. The mechanism can be opened on a hinge, allowing Weiss to replace empty vials. As a result, the weapon appears to have no tang.
RWBY Wikia: http://rwby.wikia.com/wiki/Myrtenaster