River of Souls is a short story published in an anthology entitled Unfettered.
—Release from Grim Oaks Press, the publisher of Unfettered.
Brandon Sanderson confirmed that River of Souls will include several scenes that were written for A Memory of Light, but were cut for various reasons. A video and transcript can be found here. These scenes are from the perspective of Demandred while in Shara.
Unfettered was released in June, 2013, after A Memory of Light.