Dug up old resources, here's what we had to say about it back in 05-07ish.
Kogotong - Dunmeri for "Unbreakable Brotherhood." A group of assassins eternally in the service of the prince of Hegathe. Has halls in Crown cities and Sentinel. Main hall in Hegathe.
Dunmeri for Unbreakable Brotherhood (or Guild or Faction, that sort of thing). This is the only name that could seriously be considered for use ingame, and the only faction that absolutely needs a name.
The Kogotong are the Dark Brotherhood replacement. Very close allies to the Aristosnobs, they terrify just about everyone. They originally were members of the Morag Tong living in Hegathe who, when the Morag Tong became illegal, created their own side in the schism. Rather than return to operate legally in Morrowind or join the illegal antics of the Dark Brotherhood serving the Night Mother, they became the proud servants of the "aristosnobs."
Now, they meet in secret. They're generally very darkskinned Dunmer and consider themselves part of the Crowns as if they had come with them from Yokuda.
The Mansalluh do-Tam (Knights of the Bird) is in Hegathe, but is staunchly loyal to the Temple, in particular Pashat Kabanzhir, the Curator of the Garden. They are strongly against the Kogotong, which they regard as an illegal organization, and distrust Utei Yaswa.