It's a RM that he only pays the upfront cost for. It's maintained by his bro dudes.
So a normal RM is "Pay 100 points of MP, then pay 20 MP every turn." let's say. For Iskander its "pay 100 points of MP. IH soldiers pay the 20 MP every turn instead of him."
It's also supported by them so if you kill enough of the army it falls apart prematurely too.