Lich and Apprentice

In Thaw of the Lich Lord, you can cast a certain spell called Lichdom:

This spell may only be cast by a wizard. It is a dramatic and horrifying process wherein the wizard tears his soul from his body, animates his own lifeless corpse, and then returns his soul to his new undead form. It is an agonising experience that can go catastrophically wrong, and is only practised by the most insane or foolhardy wizards. A wizard that successfully casts Lichdom immediately becomes a lich.

Models supplied unassembled and unpainted