Castellans of Dol Guldur
The origin of the castellans varies. Sometimes they are the spirits of those who were imprisoned in his dungeons, or men and elves who seeked knowledge and wisdom from him and since fell under his sway. they are spirits like though lesser to the Nazgul, formless and given shape by their cold steel armour. They were the captains of Dol-Guldur and seve him with feirce loyalty. Though when sauron returned to Mordor, there was no more room for the castellans and so they continue to stalk Mirkwood.