Basically, in the Chronicles of the Necromancer, Necromancer is another name for a Spirit Mage. Now like any mage, they can be good or evil. The gift of Spirit Mage is actually quite rare, so there are few born with it. The plot follows Martris (Tris) Drayke who has inherited his grandma's powers. Basically, Spirit Mages act as a medium between spirits and humans, often aiding spirits into moving onto the next life. They can also make spirits that are normally invisible, visible to others.
The plot begins with a political struggle as Tris' half-brother Jared usurps the throne with the aid of a vampire-like being, vayash-moru, who wants to resurrect the Obsidian King (an evil Necromancer that was defeat by Tris' grandma). The series offers a few interesting characters too, as well as later dealing more with the vayash-moru. While Tris is the main focus of the plot, his friends also play important roles too and are given attention to as well.