Second question: should it be limited to liches?
Third question: what about, for example, the Cloak of the Undead King plus gathering, say, living dead? How are they better than vampires, you ask? Well, they're linked to having the Cloak of the Undead King, and diplomacy is still a factor too.
How are they better than liches? Well, it's original - having the Cloak of the Undead King and gathering undead instead of liches... It might make for a rather tricky tactic, don't you think?..
So, what do you think after all?
Added after 3 minutes
Hmm. Looks like I didn't make it in time with my iiiiideaaa