Cylben Dolovas is a dunmer warrior, body guard and servant of Seridur living in the Imperial City. He is distrusting and unfriendly to everyone, including those who are members of the Order of the Virtuous Blood whose sanctuary is located in Seridur’s basement.
After Seridur is revealed to have been a vampire and slain Cylben becomes apologetic and says:
"I didn't know Seridur was a vampire. I swear! I serve Roland now. Please, accept my apologies for my error in judgment."