Board Thread:Lore Discussion/@comment-72.77.73.35-20140408131341/@comment-16644430-20190924071951

I don't remember any mention that Sithis claims his followers soul, only those killed in his name.

I'm not sure if merely being in his cult is enough or if some prayer needs to be said or if it's the contract being fulfilled that does the thing, but being claimed by Sithis means that you cease to exist. Utterly.

Any way Sithis is one of the original beings of creation/chaos so I guess it might actually be the only one who could successfully claim the Dragonborns soul other than Akatosh.