O-Mai was an infamous former Jeddak of Manator. Known as 'the cruel' his reign ended abruptly when he literally died of fright at some vision or visitation. The look on his face was so terrible Manatorian folklore claimed that the sight of it would drive men mad. Instead of being embalmed like other Manatorians O-Mai's body was left lying where he had fallen half on, half off, his sleep platform and the section of the palace containing his private apartments shut up with all their contents intact.
Five thousand years later Gahan of Gathol and Tara of Helium happen upon the cursed chamber and O-Mai's body. While the expression on his withered face is indeed terrible neither goes mad. Gahan is in fact so unimpressed he uses the chamber as a hide-out actually sleeping on the platform beside the dessicated remains. He later uses the chamber and the Manatorians fear of it in his plan to overthrow Jeddak O-Tar. He has E-Thas challenge O-Tar to enter the chamber to prove he is not a coward. O-Tar reluctantly agrees, but faints when he sees Gahan and mistakes him for O-Mai's spirit. This marks a turning point in O-Tar's rule as Jeddak, since his people lose respect for him.