[Baalz]

You'll start getting a handful from ritual of the five gates relatively early. As I mention, this gets you both storm demons and more demon knights, both of which are effective in small enough numbers to make this worthwhile (The frost demons, fiends and devils are also not useless, just not in the same league as those two IMO). If you run into a situation where storm demons would be very useful you can focus more on ritual of the five gates and you shouldn't have too much trouble putting together 15-20 for big fights while also making good use of the other stuff you get. Towards late game there's a good chance you've managed to scrape up the A gems to empower a vamp lord as I suggested, and just one guy casting infernal tempest sporadically can get you a lot of lightning power.

For anybody who hasn't seen storm demons in action, they're dropping AN lightning bolts with a good precision. Against most enemy troops this means you're doing a largish fraction of your storm demon numbers in kills each turn. It's not hard to figure out that killing 12-15 guys per turn is devastating against the type of super elite troops that will be giving you the most trouble.