House Ravenwatch is a noble family of High Rock based out of Ravenwatch Castle in Rivenspire. House Ravenwatch was founded by Count Verandis Ravenwatch after he left the Gray Host. The members are mostly vampires, and are currently led by Adusa-daro with Fennorian and Gwendis as Second in Command.
You can opt to join House Ravenwatch at the conclusion of the Dark Heart of Skyrim storyline by earning the achievement Taking up the Mantle, which grants you the title "of House Ravenwatch".
* Depends on your choices during the related quests.
There is one achievement associated with this faction:
||Taking up the Mantle
||Speak to Verandis in Blackreach after completing Second Chances.