Have a Siren character and return to Fyrestone in the Arid Badlands. After you kill the Mothrakk, an Eridian weapon will drop. You must search everywhere to find it. Note: To kill the Mothrakk easily, when it is low on health, shoot warning shots to draw it towards you. Kill it while it tries to get close. The weapon should be one of the following: Eridian Cannon (Berserker), Thunder Storm (Soldier) or Ball Blaster or Wave Blaster (Siren).