A Witch with the gravewalking archetype could be another minion and provide extra magical back up as well to the lord. Although defining the relationship may between the two may be a need basis where one gets someone from each other like access to more magical knowledge and rituals in return for magical support and divinations or maybe she is a consort?
Motiviation is harder and fickle to create than the actual minions. But this is what gives them a drive, a reason to go forward as well as a personality.
Other nice ideas would be evil clerics and antipaladin whoc both have archetypes related to undead. A vivisectionist alchemist with mummification discoveries can also be a nice match where he is afascinated by death and wishes to observe/learn first hand the effect of such changes.