Like the Genlock Shapechanger, the Genlock Conjurer is unique, not rare. He can be found in the great hall on the first floor of Fort Drakon. The Conjurer summons a small group of Shades, lead by a Greater Shade. Since he appears to teleport from one end of the hall to the other, use ranged weapons against this enemy rather than trying melee or eliminate the shades first.
See: Dragon Age: Origins