The Dark Hordes walkthrough
'The Dark Hordes' is a short campaign featuring the undead. Warning: spoilers ahead!
These suggestions are for Hard difficulty, but should work for Medium and Easy also.
A New Chance
Kill all enemy units. If you want to maximize your bonus, I recommend you play this scenario several times to get a feel for where the enemies are. Send ghosts to capture villages, you will need the gold in later scenarios. Start by killing the ogre and troll close to your castle with archers and Gwiti. The south-west ogre leader recruits Troll Whelps, and should probably be your next target. To speed things up, send a ghost or two to occupy the ogre's castle so it can't recruit more trolls. Move your main force north-east and finish off the orc, troll and two corpses. To finish quickly, send two ghosts to kill the north-east naga as soon as they have finished their other tasks. Use range attacks, park one ghost on the coastal village, and alternate the second ghost between attacking and healing in a mountain village.
Brother Against Brother
Kill Nati, who likes to recruit Dark Adepts, Revenants or Skeletons, and bats. One tactic is to take villages with bats and then keep your bats out of enemy attack range but ready to retake villages abandoned by enemy troops. This will keep the enemy tied down while you advance your main attack force through the forests either side of the central hills. If Nati has lots of bats flying around, recruit Walking Corpses to protect your northern villages. Use Walking Corpses to encircle opponent troops, blocking their progress and tying them down while you bring in archers and Dark Adepts against melee troops, and Skeletons against enemy Dark Adepts. If you have Wraiths, keep them out of range of enemy Dark Adepts unless you can finish them off without fearing a counterattack: remember that Dark Adepts have no melee attack. Against Revenants, Wraiths are probably your best option, and even mere Ghosts can be surprisingly effective.
The Skull of Agarash
In 0.8.3 or later, kill Nakur Kimoni, the southern orc leader. (In 0.8 to 0.8.2 you had to kill Karz Gholum, the north-eastern orc leader.) The orcs fight among themselves, which you can use to your own advantage. Use bats and ghosts/wraiths to capture villages while you recall and recruit your main force and move it south. Try to avoid moving within range of the conflict between the north-east and south-east orcs; if you stay away they will pretty much wipe each other out, allowing you to kill the southern leader quickly for a big bonus, while you mop up any stray wounded enemies with a few flying units. The Dark Spirit Leganoth can be very powerful but it is better to save kills for your lower level units so they can level up, so I usually just let Leganoth fly around capturing villages. The mountains north of Nakur Kimoni's castle are where most fighting usually happens. The cave floor in the eastern part of the mountains is slow to cross, so I suggest moving through the pass with support from units in the mountains flanking the road. Remember to guard your Dark Adepts from enemy melee units. One way to do this is to attack an enemy Orcish Grunt with several of your Adepts one after the other, which should finish it off. Then move a melee unit where the Grunt stood, shielding your Adepts (you will also need at least two other melee units to complete the shield).
Mages and Elves
Move Gwiti to the tower, or kill both enemy leaders. On Hard, the enemies have large incomes even if you capture most villages, so getting to the tower quickly is advisable. Very difficult on Hard prior to 0.8.4, as the enemy mage recruits paladins that dominate your corpses. In 0.8.4 or later, recruit enough (at least 70-80 on Hard) Walking Corpses to surround each enemy unit with 4-6 corpses, using packs of ranged troops to finish off enemy melee troops at night. The extra 2hp for corpses makes a big difference here! You can use fewer corpses, but then you need very careful placing to ensure they form an effective barrier (remember to space them with at most two empty hexes between any two, so that their zones of control have no gaps). Use ghosts and bats to capture most villages and reduce enemy income. Harass the elves using flying units, corpses, and possibly some ranged units, while eliminating the mage's defenders one by one. If the mage leader vacates its castle, just clear a path to the tower along which you run with Gwiti.
Inside the Tower
Find Crelanu's Book, battling mages on the way to the library in the north-west corner, through a secret passage. You cannot recruit or recall, but four Revenants arrive to help you against 4 Mage, 3 (4 on Hard) Red Mage, 4 Spearman and 2 Swordsman guardians, which are luckily scattered in small clumps. To keep all revenants alive (and to see a small Easter Egg), finish off enemy mages with melee attacks before they get a chance to use their range attack. Gwiti should be at least level 2 by now and can drain mages and spearmen to stay alive, and may level up in the process. Even the level 1 mages are very dangerous against your Revenants, so use Gwiti as your frontline -- the mages will be hesitant to attack him with ranged attacks, and he can just drain the enemy mages to death. There are lots of chests with gold, as well as three potions to imbue melee weapons with a cold attack. The secret passage to the library is guarded by a slightly weakened Great Mage.
(Drake Raider added this note: Deathmaster [Level 2] cannot drain; but the Demilich [Level 3] can, and A New Chance is good for geting Gwiti advanced to Deathmaster, just try to advance him to Demilich before this scenario.)
Take the book (in the clump of forest in the north-east) with Gwiti. Move Gwiti to the castle in the swamps to reveal 36 castle squares from which you can recruit Walking Corpses (but nothing else, and no recalls). Two or at most three castles of corpses should be enough to build a passage to the north for Gwiti to travel along. Keep your corpses in a line to maximize their chance of survival, and rotate corpses that are wounded in one round (but survive) away from the frontline. The enemy really has no chance here, 72 Walking Corpses only cost 360 gold which you should easily have in reserve from your previous exploits, and take a _long_ time to kill.
Creator's Notes: Enjoy the B-movie horror show of a million marching zombies.
Kill Leonard, then move Gwiti to the end of the mountain pass. The south-eastern castle and surrounding hexes teleport you back to your starting castle. Initially this castle is empty, but after Leonard dies, Lord Aretu appears here. No early finish bonus. Aretu has lots of gold to flood the plains with dozens of paladins and white mages, and is very unlikely to leave the teleport squares around his castle, so probably can't be killed. This should encourage you to finish quickly, though pay careful attention to time of day, your army is very powerful against Leonard's army at night but very vulnerable during the day.
Gwiti is slow in hills and mountain terrain (4 movement units for every hill or mountain hex), so for a low risk strategy you may want to position Gwiti close to the north-east mountains while you kill Leonard with ranged attackers. Since it is difficult to keep any of your units alive while you flee from Aretu, I usually recruit only fresh units, including lots of Walking Corpses (at least 400 gold worth of units in all). Then you can finish off many of Leonard's troops with Gwiti on your way to his castle, and gain a final experience boost by finishing off Leonard with Gwiti also. If you do this, be prepared to sacrifice most or all your army to slow down Aretu's paladins and mages while Gwiti runs for the hills. The paladins can't cross mountains so Gwiti is safe from them once he has made it that far, but Aretu's White Mages have no problem pursuing you in the mountains, and those that are quick can catch up very fast. A lost orc, Tanar, will join you as you finish the scenario.
Move either Gwiti or Tanar into the water at the north end of the tunnels. There is a chest containing 320 gold past the north-west leader, but also a poisonous giant spider (use a Wraith to kill it), lots of ulfserkers that fight until death (again, use a Wraith), and many twisty tunnels where you must travel single file. It's probably not worth recruiting many units, a single Wraith is probably sufficient.
In version 0.9.1, it is sufficient to move any unit to the topmost water, not just Gwiti or Tanar.
The campaign ends here for now.
Sneak Peek - Underground River
Exit the mountain, and try not to get caught by the flood, the nagas or the dwarves!
When will there be more? I can't wait to know the fate of Gwiti. Will he die in the end? Or will he find redemption and become human again?