Special weapons report #10 from the Office of Vanguard Intelligence.
The Susanoo is a legendary fusion rifle that is a semi hidden quest from Banshee-44. The last step of the quest is to breakdown 10 points worth of rare blue or legendary purple fusion rifles. Once the quest is complete you are rewarded with the Susanoo. It starts as a 280 solar fusion rifle. Once it's completely unlocked to can take on the energy of the subclass character that is wielding this weapon. This makes it better and similar to the Murmer from the Dark Below.
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My only concern thus far is that it never switches back to it's original element, unless a short time is considered 15-20 mins.