Neromancers are those atheistic dark wizards everyone has heard of: they disturb the dead to inflate their ever growing army of dead. Necromancy.
However, a Necromanccer is not completely limited to Necromancy. Necromaners are also skilled in other forms of Dark Magic, and sometimes they also get to learn the ways of Destructive Magic. After all, it is vital to know how to defend yourself when your army of skeletons fails the day.
A Neromancer can either work alone, or work for the Republic of Keldorn.
Necromancers may be evil, but not stupid, and certainly crazy - well, maybe just a little...
They wouldn't go so far to ally with demons, as their hatred with demons and anything demonic dates back to the Demonic Invasions.
Necromancers are a prestige class of Magicians, restricted to characters of Evil alignment.
Being a Necromancer comes with an advanced knowledge of the Ancient Language, the ability to turn the dead into undead. After attaining a certain amount of power and knowledge, they can turn back dead bodies back to life in their original form, given that their bodies haven't decayed yet - but most necromancers do not bother with it.
Unfortunately, due to technical limitations being a Necromancer in the game does not come with the ability to raise the dead in the Necromancer way, but it does automatically give the player curse and poison spells - just like Dark Elven Clerics - and allows the player to continue learning spells from scrolls, just like Magicians do. Necromancers also have access to the Resurrection spell, allowing them to revive dead allies, a spell only Clerics have other than them.