Horak-Mul made his debut voice acting and 3D appearance during the Inquisitor Story of SWTOR, predominantly on the planets Hoth and Voss. While many of the ghosts that the PC Inquisitor/Kallig consumed were constructive, Horak-Mul stood out as a truly evil/antagonistic force that would challenge your character until you completed the healing/binding ritual towards the end of your player storyline on Voss.

However Horak-Mul actually originated as a deep cut ’90s secondary character within the Tales of the Jedi Comic Series by Dark Horse, where he was present in the ‘Golden Age of the Sith‘ issues 3, 4 and 5.
In this series, Horak-Mul was seen as an arrogant, mongering Sith that vouched loyalty Ludo Kressh as he waged war against Naga Sadow to be crowned Dark Lord of the Sith. He was most notably seen wearing a gold covered cybernetic eye that would later come into the possession of Celeste Morne.
He would only be mentioned once more, briefly in the 2008 “Knights of the Old Republic” comic series Issue 25.

Horak-Mul was killed in Issue 5 when Naga Sadow commanded the Massassi warriors on Horak-Mul’s ship against him.
After he was buried, his body would be excavated centuries later and placed upon the Star of Coruscant, which would eventually crash on Hoth where much later the Inquisitor, Kallig would find him.

Unlike the other ghosts that wish to be freed once the Inquisitor Kallig defeats Darth Thanaton, Horak-Mul wishes to remain bound within your conscious as that would prevent him from ever returning to the tether point of his tomb amongst the wreckage on Hoth. It is player choice whether they free him or allow him to stay.