The necromancer first appears when you cross the bridge towards the Tower of Ishal. The Warden encounters it right after seeing Cailan's crucified body and it summons several skeletons. Once the Warden tracks it down to the Ostagar battlefield, it reanimates a Risen Ogre (Elite boss), as well as constantly spawning skeletons. Killing the Necromancer will destroy all reanimated creatures, including the ogre.
See: Dragon Age: Origins