When a shard dies, the souls of those people merge with their equivalents on the first, but what if the void can't be merged? Those souls were transformed. I have a theory that when the warrior of light utilizes the reaper job, they actually tap into the version of themselves that is within the void.
The thing though, is Cuchulain. We know that Cuchulain was the warrior of light on the 13th, so I don't know how this could work. Thoughts?