ToME: the Tales of Maj'Eyal

I have learned recently that you cannot invest more points into lichform if you are undead. Not just straight up undead-undead, but rather treated as undead via a belt/lantern of unlife. Don't know if this was the exact cause, but I thought I should post this here in case anyone else has a similar ...

As the title states, I'm playing a necromancer (cornac) for the first time and can't for the life of me figure out how to become a lich! I already completed the ritual at the sher'tul fortress and have one point in lich form. The problems - The sustain ability is greyed out so I can't activate it de...