My sincerest apologies for the late reply to this thread. I was made aware of it months ago, and yet I fear it slipped my mind. But better late than never, I suppose.
If you're looking for a Dark Brotherhood Guild, then we in the Blacklight Sanctuary may be of potential interest to you. We're a literal down-to-the-book interperation of the Dark Brotherhood, and we follow the lore surrounding it rather religiously. Though as the name suggests, we're stationed in Morrowind, and not near High Rock.
Most unfortunately, the two Dark Brotherhood guilds which laid stationed in High Rock (Wayrest and Balfiera respectively) are no longer active. But if your character is not afraid of Dark Elves and volcanos, then you may feel free to shoot me a PM here on enjin so that we can further discuss the details.