The Necromancer is a hostile NPC Boss that must be defeated in order to obtain the Deathshard. While unconfirmed, he has likely created the Skeletal Warriors, for an unknown purpose that may be revealed later.
The Necromancer is hostile, and will attack on sight. Unlike most enemies, the Necromancer is stationary, and will not chase you upon leaving.
The Necromancer uses Fireball on you when at long range, and a spell of unknown name when you get close. The second spell is incredibly damaging, forcing you to stay out of range. As such, the Fireball spell is effective against him. However, you want to ensure that your spells do not collide with his, or they will explode and your spell will not reach its destination. Dodge his spells while casting yours and he will eventually be defeated.