Greater Ghoul is a stronger version of the Ghoul that can only be found on levels 3 and 4 of the Labyrinth under Durlag's Tower.

Aside from all-around higher stats Greater Ghouls are also permanently hasted, making up for the slow speed which was always one of the greatest weaknesses of their regular cousins - though it should be noted that even a hasted Ghoul only moves as fast as a regular human(in Enhanced Edition/BGT/Tutu) Just like normal Ghouls they attack with both claws and bite, which can immobilize the target on hit.

Assuming Grael speaks the truth, it can be speculated that Greater Ghouls are what remains of the party led by Durlag Trollkiller that fought to originally subdue the demon Aec'Letec, all turned undead because of his Death Gaze special ability.