House Goratrix is an organization within the Tremere clan that was developed by Goratrix himself. Originally founded as a cultish conspiracy under Goratrix during the Dark Ages, House Goratrix later became the backbone of the Tremere Antitribu when Goratrix and his followers defected to the Sabbat.
House Goratrix is believed to be a mockery of the Pyramid of the Tremere clan that Goratrix had co-founded. Like all Tremere antitribu, they are regarded with suspicion and sometimes hatred, and share the curse of the Tremere clan. In addition to regular Thaumaturgy, the Tremere antitribu of House Goratrix often study Dark Thaumaturgy.
The Sabbat makes no distinction between Tremere antitribu, but similar to the division between the Country Gangrel and City Gangrel, there is a division of sorts between the Tremere of the Sabbat. The difference is not particularly profound compared to the Gangrel division, though.
While the antitribu of the Americas were a product of the 18th century wars that rocked the continent for half a century, the House of Goratrix developed somewhat earlier in Europe when Goratrix, one of the original Council of Seven, turned traitor on the Tremere. Their habits are somewhat different; in Europe they maintain their own chantries outside of Sabbat temples, whereas the Americans cordon off chunks of temples for labs.
Also, in Europe the Lasombra are far more prominent than in America (most Sabbat Tzimisce being located in the New World), so that the Tremere in general are more powerful. The primary competition is first with the mainline clan, and second with the Old Clan Tzimisce, who make no distinction between mainline and Antitribu.
The organization of House Goratrix is a bit more formal than that of the American Antitribu, somewhat reminiscent of the Hermetic structure of the original clan. Goratrix sits at the top, ostensibly over all Tremere antitribu (though the Americans are too autonomous to really recognize his authority). There is a Council of Six, a number chosen to parody the beliefs of those who swear by numerology. Below that, there is a Jesuit-like rule of obedience to sire and pack.