Blood-Ulm, or Ulm with lots of scouts harvesting blood slaves, makes for one mean devil-factory through forging Soul Contracts. Takes a while to get going unless you have a blood pretender to forge the initial contracts and make blood enhancing items, but once you have a decent blood income it isn't all that expensive to empower a smith to L3 in blood and boost him to L5 with items (Armour of Souls, Brazen Vessel), give him a dwarven hammer, and forge Soul Contracts for 40 blood slaves apiece.
It can even work in MP against the unwary opponent who does not consider Ulm a blood nation, if you just make sure to hide your devils really, really, well until you have a high production going.
