The Covenant of Trombli was a sect of priests representing the Wolf, Ram and Hart in Pylea, thus they were Pylean branch of Wolfram & Hart. They were led by the ambitious Silas and occupied the top of the Pylean social hierarchy, making Silas the de facto ruler of Pylea.
The Covenant controlled a half-demon, half-human warrior named Groosalugg, and used him to enforce their will through physical might. However, the Covenant had another, more sinister use for Groosalugg. It was prophesied that he would perform Com-Shuck with an entity bearing "the curse". The entity turned out to be Cordelia Chase. Had the two mated together, thus consummating the com-shuck, the visions sent by the Powers that Be to Cordelia would be transferred to Groosalugg, and they ultimately fall under control of Wolfram & Hart.