The cult of the Harvesters of Eyes was led by Vera Gemini, who was half human and half demon. She led them to bring about Xenogenesis, in which a race of demons would come back and conquer Earth. She managed to gain possession of Doctor Strange's Eye of Agamotto in order to do so.
They succeeded, temporarily, and the demons were brought to our plane. They killed all the cult members as they arrived. Gemini manipulated the demons and took control of them herself. They were eventually defeated by the Defenders and returned to their dimension.
Description of the Cult
The demons' plane is almost all matter, and no empty space. One of their leaders is Belathauzer.
The cult is very hierarchical, and everyone has a rank. One such rank is Agent of Fortune. Another rank is Reaper of Souls, or assassin. They trained Devil-Slayer to be their Reaper, but he rebelled and later fought against the group.