Seeker

"Seekers are servants of Hermaeus Mora. They are rarely seen outside his Oblivion realm of Apocrypha."

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Seekers are Daedra that inhabit Hermaeus Mora's plane of Oblivion, known as Apocrypha. They typically serve Hermaeus Mora, but Miraak took control of some of them during his stay in this realm.

Locations
Seekers can be seen throughout Apocrypha, usually guarding tomes of forbidden knowledge. When idle, they usually conjure a book from thin air and begin to read.

Loot
Seekers usually carry two books and a scroll, and occasionally you can find Spell Tomes on them.

Combat
Seekers use a shout like attack called Seeker Drain, as well as a damaging spell. When entering combat they will summon a second, weaker seeker to aid them.

High Seeker
These are the seekers with Miraak when you first encounter him.

Trivia

 * There is a spell called Conjure Seeker used to summon a Seeker from Apocrypha. This spell also works in the said realm of Oblivion.
 * The seeker look similar to Cthulhu, from Lovecraft.