[Morgeruat]

Liches can gain further understanding of magic, in the form of learning new spells, devising spells of their own, and basically gaining a further understanding of the nature of magic, even though they cannot increase their potency with those spells (raise levels) when they've reached a certain understanding of magic and it's universal applicationthey can become demiliches, when the body falls away and becomes nothing more than dust and only the skull remains while the soul is off travelling other planes.

(i think if you add a lich to your party they will be able to level up normally) despite PnP rules to the contrary, and a mage must be at a minimum level 18 to become a lich(unless the ravenloft Requiem rules are being used) so he would have access to level 9 spells.