Great Tom.
I think your about right for the spell’s ether cost, perhaps 6… I would however think about rolling a D20 once the Harbinger is summoned. The score would equal the amount of souls it would take to fill its quota and it would disappear.
Also, I feel the spell needs to have a little bit of a backfire element to it. As a Necromancer is alive and a Lich has cheated Death, so their would always be a chance that the Harbinger would turn against its new master. I would Propose a battle of wills. So both the Harbinger (2 Soldiering) and the Necromancer would roll off their soldiering values against each other. If the Necromancer fails… then he or she would be in a lot of trouble…