Sorry I've been thinking on it. It is just a bit hard to gauge right now as immortality is something nigh impossible. the best shifters in the world can use advanced healing magics to basically turn back the biological clock and stay young, but that has a limit. Demonologists trade for it all the time, but it usually is part of a trick or has a terrible cost.
Now while this seems like a masssively unpleasant situation to be in I think the number one thing I need to know is how his immortality works First and foremost, and i assume you know this by now he has to die to normal things. he just can't die of old age. Besides that it is hard for me to understand how this functions.
When you say lich I can only think of advanced necromancy which is highly frowned upon by the world at large. minor stuff like talking to ghosts etc is fine, but lichdom is basically shedding your mortal coil to become a necromatic abomination (what these guys usually would go kill).
It does bring up a good point though. That is Seph's specialty He is a mage hunter as in he kills mages who go to far and are considered a threat to the world, the balance, or something just as important. perhaps we can find something there to explain this situation.