In the event that you are dealing with 5E, I would recommend looking at the Flameskull (MM p. 134).
Blazing green flames and mad, echoing laughter follow a disembodied skull as it patrols its demesne. When the undead flameskull discovers trespassers, it blasts the intruders with fiery rays from its eyes and dreadful spells called up from the dark recesses of its mem9ry. Dark spellcasters fashion flameskulls from the remains of dead wizards. When the ritual is complete, green flames erupt from the skull to complete its ghastly transformation
While it isn't a wizard that has achieved some form of lichdom or un-life, it's unlikely that a wizard potent enough to overcome death would be suited to battle a lower-level character. If you're hell-bent on having them encounter such a creature, I would recommend that you send them against a demilich or a lich that has somehow been weakened, wether it's by having its lair blessed by powerful clerics, or by having part of its phylactery partially destroyed.