Is it the right assumption that you shouldn't be able to play as Regelus for the forces of darkness? If so, I've found bug, as I'm currently playing him for the forces of darkness.
I think the problem is that I had already selected Regelus in skirmish as my demigod. Then when choosing forces of darkness Regelus was already selected as my Demigod. If I try to select a new Demigod the "light" demigods are greyed out including Regelus.