Greater Doppelganger are a stronger version of doppelganger that are capable of casting spells. They will cast Mirror Image and Haste before attacking. They start appearing as fixed spawns in main storyline from Chapter 5 onwards and can also be found on Labyrinth Level 1 and 2 of Durlag's Tower.
Also on Level 2 of the Labirynth there are "special" versions of Greater Doppelgangers. Rather than casting Mirror Image and Haste, they can instead change between forms of Durlag Trollkiller's family: Islanne, Kiel, Fuernebol - seemingly at will - and inheriting abilities and equipment of each. They can also cast Cure Light Wounds.