The Chthonians. It is believed they were responsible for the neutrino emitting machines found at site JENNIFER MORGUE. Per Angleton, they are “polymorphous, occupy areas of the upper crust near the polar regions, and BLUE HADES are terrified of them.” It was noted in the Jennifer Morgue that Dagon is a Chthonian. Per The Fuller Memorandom, DEEP SEVEN are “the lurkers in the red-hot depths”. (First mentioned in The Jennifer Morgue.)
DEEP SEVEN is believed to have long standing issues with BLUE HADES, none of which involve humans.