Looming atop the great mesa like a dark thundercloud, between the ruins of Al-Merayah, the Summoning Place, and the great Sinkhole, High Sheidal was founded by the Pictish exile Shalkana. Driven to worship of Yog by strange visions from her earliest feverish days in the Exiled Lands, she spread the faith at spear's tip and cleaver's edge, cutting a bloody swath from the southern river to the slopes of the volcano to the Isle of Siptah's shores, the dark kingdom of Lastyr.
High Sheidal is the heart of that vile realm, and the great Cathedral is the heart of High Sheidal.
Other faiths are tolerated (or at least ignored) within Lastyr, namely the followers of Ymir and Derketo, and Dagon acknowledged as a peer, but Yog holds dominion over all. As a being from the Outer Dark, beyond time, space, and understanding itself, he is held as a true god amidst a pantheon of aspirants and pretenders. Set and Mitra in particular are considered divine impostors and their infidel followers are favored pitfeed, as are any that would deny the primacy of Yog.