Where is Demonis?
 

I've cleaned Ellinia and Death Lands (except for the red dragons), defeated Karador and been given the quest to find the key etc. So, where should I head to? Where's Demonis and how can I reach it ? Thanks!

demonis is accessible from the underground sea, which itself is accessible from taron mines. Taron mines is the area underneath the dwarven capital. Remember the castle that the dwarf king wanted you to recapture? Well, head along the path infront of it in the opposite direction from the way you came to it. You will find an elevator. Take it down to the underground sea. You should have no problems finding demonis from there

Ok, got it. Just another question : which units are more useful in Demonis?

Probably those resistant to fire but that don't do fire damage themselves. Anything that has extra bonus' vs. Demons etc. Pretty obvious really :)

Yeah but since it's my first time playing the game, I don't think I really know which units are resistant to fire or have a bonus against demons. Any suggestions? Thanks.

Knights, Horseman, Dragons, other Demons, maybe more. Just don't be using plants against them :P

Dragons ? very bad choice they do fire damage ...

Meh, I'd say forget about the previous suggestions concerning fire resistance. Just don't take units that deal fire damage, such as Red and Black dragons.

Do you have level 3 Distortion Magic? Hopefully, you do, and you'll have leveled Magic Shackles to level 3. Demons are basically walkers/melee fighters who have some really nasty special abilities, mostly the Demoness' Infernal Exchange (teleporting one of your vulnerable units to the midst of enemy ranks) and the Imps and Scoffer Imps' fireballs; these are all shut down when you cast MS level 3. For a limited time of course.

So, my advice to you is, get Elves and Hunters and shoot the demons down with a combo of Magic Shackles and Slow, then engage them with Sprites and Lake fairies, preferably while your hero is wearing the artifact Anga's Ruby which makes she-fighters overpowered (in my opinion). Finally, you could fill the last slot with, say, dryads, and put those (scoffer) imps to sleep using lullaby, then in the next round cast a spell that raises the elves' initiative, then summon some thorns, etc.

Of course, the above is just my ideal combination (not really with the dryads, though, usually I will have Emerald Green Dragons), there are lots of other possibilities and strategies. Good luck!

Thanks...just a last question : should I try to clear Demonis from all enemy troops or should I just finish the key quest (Nexy Caslte) and the Roads Architect quest and go back to Ellinia to enter the Labyrinth?

I was just listing units with fire resistance. Also Emerald Dragons do physical damage and are fire resistant.

If you can handle it, kill everything. It's all xp.