1. The Devil's Outpost is in Kriegspire, just north of the extinct volcano. Your party will probably be attacked on its way by assorted earth elementals and minotaurs, so you may want to rest up and heal before taking on the devils. If anything gets broken you can get it mended in the village, but try not to lose any of your characters, because there's no Temple. (There's the curator in Castle Kriegspire, but believe me, you don't want to go in there yet. 
Besides, it's expensive.)2. Before you can become a master in Dark Magic, your party has to have a notorious reputation. There are various ways of getting there, including selling human bones (check out the houses in Rockham, the village west of Free Haven - find bones with the cannibals in Bootleg Bay and in Wromthrax's cave in Mire of the Damned), and taking advantage of the Face in the Superior Temple of Baa. Killing peasants also works, but I treat that as a last resort.
A warning before you do this: it's a lot easier to get a notorious reputation than an angelic one, so it you want to achieve mastery in Light Magic, do that first. Once the quests that boost your rep are done, you can't get any more.
3. Skills for your knights: Repair is always handy; as you get on in the game, you'll find more and more opponents who can break things, and it's a pest having to go to town and quite possibly wait for the shop to open. Also, one of your characters will need at least expert in Perception in order to open the doors in the Superior Temple of Baa - and there's one door you won't be able to open even then.
You best weapon combo for one knight is the spear Poseidon in the right hand, and the sword Hades in the left. Be careful, though, to carry Hades safely, you need an item of Regeneration, otherwise it drains the character using it.
Hope this helps - get back to us if you need more help. 
