Sedos was not an Assassin when he was alive; he was a successful businessman with a beautiful home and a loving family. But it all came to an end when he was murdered in cold blood, and an intense bitterness regarding his untimely demise still festers in what remains of his soul. Necronumos exploits this negative energy, using it to drive Sedos into taking revenge on the living.
The most difficult thing about deploying Assassins is keeping them alive (or alive-ish in Sedos’ case) once they’ve made their initial strike and are deep behind enemy lines far from your healers. Sedos has found a unique way around this problem. He drains the life force of his target and absorbs it into himself, and the bigger the target, the more life he can steal. So with a bit of careful planning and more than a little luck, Sedos can revitalize himself with each kill, and thus keep fighting longer than most of his fellow Assassins.