Nether mages rely on two resources - mana and population.
This is a potential strength as Black mages can generally support a larger army than can a mage who relies on one single resource. Once Nether mages have researched their spells, they should focus on maximizing mana and population production (that is, have lots of nodes, towns, and farms). Since money is not an important resource, nether mages should build lots of fortresses (2,5%). This will improve their defensive abilities and add extra residential space.
Since nether magic is closely connected with Phantasm and Eradication magic, I suggest you look to all three specialties for spell and item combinations. Nether mages may enjoy success with the Satchel of Mist/Fog Cloud combo (see Phantasm page for detailed explanation), or with several eradication spells, including Disintegrate. Gaze of Death is a killer against Eradication mages. Known effects are killing large amounts of Red Dragons before the battle even starts. Ash of Invisibility will let your numerous Zombies and Skeletons attack before they are slain by the enemy.
If you are on defense use Black Sabbath (use it when you are on the offence as well as it gives you a lot of creatures, not to mention mana) to gain Zombies and Skeletons as your main stacks. Then use Lich as you third stack and an off colour as your 4th and 5th stack. I have used these stacks for a while and have not lost a single acre to attackers. 350.000 Zombies, 200.000 Skeleton, 1000 Lich, 750 Mind Rippers and 4500 Salamander.
If you on the other hand are going on the offensive try to get your Liches up to first or second stack together with your Zombies. Use Salamanders as your third stack and Skeletons as your 4th. Underestimating the Wolf Raider unit might be a big mistake... if they remain behind the Liches for cover they can deal serious blows to whomever might be foolish enough to oppose you.
Remember to keep an eye out for your population and never go for a higher ratio on towns to farmlands more than 1 to 3. If you do it will reduce your population. Run your country on balance when it comes to people, - 10,000 when it comes to mana and + 10,000 geld. This is normally the way many a Nether mage runs his land.