Seekers are servants of Hermaus Mora, tentacled horrors of Mora. Pretty creepy right? After reading this little passage, these things give me chills.
"Morian Zenas described to me an endless library, shelves stretching on in every direction, stacks on top of stacks. Pages floated on a mystical wind that he could not feel. Every book had a black cover with no title. He could see no one, but felt the presence of ghosts moving through the stacks, rifling through books, ever searching."
http://images.wikia.com/elderscrolls/images/3/35/Seeker_Reading.png
That thing is doing what is described in the passage? Are seekers corrupted mortals, that were lost on there pursuit of knowledge and dark secrets?