Fused
The Fused are ancient singers who survived their death and are able to possess living singers during the Form bonding in the Everstorm. They are the highest authority in the singer society. They have powers similar to Surgebinding, presumably this is Voidbinding. Moash mentions that they only have nine orders instead of ten.
Life Cycle
Fused exist Almost like a Spren until they Bond with a Parsh. This Bonding is different from a normal bond and apparently obliterates the original mind of the Parsh that they inhabit. Each Parsh has a gem Heart and this is were a fused exists inside the parsh. When Parsh inhabited by a fused dies they return to the Cognative realm. In the past they were returned to the prison on Braize, now each passing of the Everstorm allows the fused to bond with a new Parsh. A Parsh can resist the bond.
Powers
The fused all have forms of power but it is unknown if each fused has only one form or if they can change forms each time they are reborn. Thus far the one fused seen to have reborn did so into the same form of power. These Forms of Power have Similar abilities to Surgebinding but runs on Voidlight. Notably we see flying as the most common type or at least most visible. In addition to that we have Lightweaving used in an attempt to steal the Honor's Drop from the Gemstone Reserve, and even a Fused using Edgedancer gliding.
Notable Fused
Speculation
It is Possible that part of what allows a fused to live on and have a form of power is that their mind was "fused" to the Spren associated with that form of power.