Sneaking through Demonis was a ton of fun. I managed to pick up all the important spells in the game that I was missing except for Phantom. I have absolutely no magic crystals and a long line of spells to upgrade, but at least now I can calculate how many magic crystals I need to get the spells I want.
I also picked up some very nice artifacts in Demonis. I got a Dragon Tooth Necklace to replace my +4 Defense stone egg thingy. The necklace is +3 Attack, +2 Defense, and +1 INT, so a big upgrade there. I also bought the Sword of Equilibrium from Xeona's castle and completed Isshara's quest to get the whip. The +attack to Demonesses isn't very important to me, but the +1 Fire Damage is very interesting. If you are using low leadership units in large numbers that +1 damage ends up being very powerful. For instance, your peasant stack would do an additional 3000 damage per hit, or your sprites would do an additional 1800 damage, etc. Not very useful for Giants and Dragons though.
I have leveled up the Reaper's Time Back skill so that I can turn back the clock on level 1-4 units for 25 rage. This has made fights so much easier. Of course everyone knows that you can put a powerful unit in harm's way and then restore 1.999 turns of losses with Time Back, but it also resets any charges you used in the last 1.999 turns. If my Inquisitors get hit, I make sure to use their Resurrection ability before I Time Back so that I can use it again later in the fight. I'm sure as more powerful charge based units come along this will be a handy trick.
Demonis has some interesting recruits available:
Demonesses, Demons, Scoffer Imps, Cerberi, Fire Spiders
Druids, Black Unicorns, Sprites
Necromancers, Black Knights
I was tempted to recruit some Druids, but I'm really stomping through the Dwarven lands with my current army so I'm not sure I should switch anyone out. At level 19, the Xeona castle fight with no losses was impossible so I'm going to clear out Mehgard and come back. I am just now beginning to learn the pain of switching out my weapons depending on what the enemy stack contains. If they have dwarves, I can't use the dwarven hammer, if they have humans/elves I can't use the sword of equilibrium, if they have undead use the sword of equilibrium, but if they also have humans then the silver rapier. Annoying.
It'll only get worse when I have 3 weapon slots.