Seridur is an altmer nightblade, vampire and head of the Order of the Virtuous Blood in the Imperial City.
The Order of the Virtuous Blood is dedicated to rooting out vampirism in the imperial city and Seridur claims to have found one named Roland Jenseric who he witnessed bite the neck of a women in the night and he asks you to find and kill him.
Upon discovering Roland Jenseric where abouts you find out he is not a vampire and it was he who witnessed Seridur bite a women. You'll then have to find Seridur in a vampire strong hold and put an end to him.
"What better way for a vampire to hide than among vampire hunters?"