River of Souls is a short story published in an anthology entitled Unfettered.
History[ | ]
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.