DEVILS O' RAMA
For everyone else that wants devils but who don't get Demonbred mages... there is my old friend the Soul Contract.
Do whatever blood hunting that works for your nation.
You will need a blood-5 mage and Construction-2 to make Soul Contracts.
It is a cursed item and it will horror mark the character you give it to.
Your goal is to get at least one if not more contacts made per turn.
The contracts produce one devil every turn.
Each contract costs 80 blood slaves.
Devils have a summoning cost of 7 slaves, so effectively it takes 11 turns for the contract to "pay off". After that it is effectively an income of 7 slaves per turn (in the form of a devil). It takes a while for the production to really take off. During this wait time you will have spent resource and won't have reaped benefits. This is the dangerous time for the Soul Contract strategy. It's a bit of a long term investment. If you can afford the time to ramp up for the payoff it is awesome. If you need power fast, convert your slaves into something that will get you power faster. Consider things with care when making long term plans.
Also keep in mind that you must have blood or death mages to lead the devils. This can be difficult for some nations to cultivate.
On the other hand, if you do manage to crank out a bunch of contracts and have the leadership... you will have a never ending stream of very potent troops. That can't be bad.
You might consider forging a Dwarven Hammer. This will allow you to make the contracts at a reduced cost. Don't ask me how a hammer helps make a blood soaked piece of parchment. It just does. Don't go crazy trying to get a hammer if it's hard for your nation. It's more important to expand your economy and work the blood magic. But if you can squeeze it in... get one.
Strangely Ulm has a major advantage when working this strategy. The master smiths start with a forge bonus, and can increase it further with a dwarven hammer.
Also consider that many players like using the Prince of Death pretender with Ulm. A POD with some Ulmish armor can be used as an early game Thug. The POD also is a flyer and can lead many devils. A shard of syngery for Ulm? You be the judge.