Story help sought -- Necropolis / Disease Demon

It gives the equivalent of the Unaging Virtue, as well as some additional bonuses for dismemberment. It also will cause lots of warping. It is a mystery virtue.

As Khelek says, not of itself, although the unaging virtue prevents you from showing the effects until you fall over - you'd have to stack it with something else. Unfortunately the Gruagachan arts aren't great for resisting aging, particularly as you can't Bless yourself (without an enchanted tattoo, but the aforementioned sub-tradition can't make those).

Sounds like the best way is to have a backstory where a magi (or necromancer) uses a a modified External Soul Virtue (perhaps infernal realm) that causes wounds or warping the way you want it to and modify the nature of the soul jar a little. Then when he figures it out, makes himself a Supernatural creature by achieving a Might score. This basically freezes him as he is, with some additional qualities as needed.