Greater Doppelganger's are a stronger version of doppelganger that are capable of casting spells. They will cast Mirror Image and Haste before attacking with a battle cry such as Time to fillet some MEAT!.
They first appear in Baldur's Gate as fixed spawns in the main storyline from Chapter Five onwards. In Tales of the Sword Coast they are found on labyrinth level 1 and labyrinth level 2 beneath Durlag's Tower.
On level 2 there are "special" versions that will change between forms of Durlag Trollkiller and his family: Islanne, Fuernebol, and Kiel the Legion Killer - seemingly at will - inheriting the abilities and equipment of each. They will cast Cure Light Wounds upon themselves and their brethren during battle. Those that imitate Islanne will also cast Shadow Door, Horror, and Lightning Bolt.