The Brotherhood of Seth (also known as the Temple of Sethiete) is a temple of worship for the deity known as Seth. They were prominently active throughout the Imperial Simulacrum in city-states such as Camlorn and Silvenar, or smaller settlements such as Ein Meirvale and Stoneforest. In that time, the brotherhood held vast power over the city-state of Gideon, on the western corner of Black Marsh.
In the years prior to the Simulacrum, the Imperial Battlemage, Jagar Tharn was once a mage-priest of the brotherhood and stole valued information, such as powerful spells and information regarding the Staff of Chaos. In some matter, Brother Barnabas of Tethis had learned about Emperor Uriel VII's capture at the hands of Tharn and suddenly went mad for justice. Senior Brother Halfas Varn and the other priests tried to calm him down, but in the next day, he killed Brother Martinus and fled to the Mines of Khuras, taking a scroll that could reveal the location of a piece of the Staff of Chaos, the Crypt of Hearts.
Halfas Varn was later approached by the Eternal Champion, who was seeking the missing piece. They were sent to the Mines of Khuras, to hopefully retrieve Brother Barnabas and the map, in exchange for the location of the Crypt of Hearts. The warrior discovered Barnabas' body deep in the mines and was able to recover the map for the brotherhood. Halfas Varn was able to use the map to decipher an Elder Scroll that revealed the dungeon's location.