The Eclipse
A group splintered from the Mad Sun-King’s Kestrels and Sun-Priests, the Eclipse was assembled by Sylens on behalf of HADES to unearth the ancient FARO war machines. While HADES claimed to be the Buried Shadow of Carja myth, it promised the unearthed machines would be under the control of the Eclipse, and crucial in their efforts to retake the Sundom. This convinced Helis and Bahavas, who went on to lead the cult, excavating a number of Deathbringers and Corruptors, ultimately marching on the Spire at Meridian.
The main base of the Eclipse was buried in the jungles of The Jewel, on the borders between the Carja and the Tenakth. It was raided by Aloy and suffered significant damage during her escape.
Canonically, it is not clear whether the Eclipse survived the attack on the Spire. Leaderless, including Helis and Bahavas, with HADES also defeated, it is plausible that it disbanded … but it is also just as plausible that HADES’ showing of overwhelming force at the Spire only strengthened their resolve.
The Eclipse in Skyline
While it is implied in Horizon canon that all Eclipse are also Shadow Carja, Skyline relaxes this. The mythos of the Buried Shadow might have been believed by Carja members, but it’s just as plausible that Tenakth would have tempted by the power of the unearthed machines, Oseram by technology, and even exiled Nora if only by the sense of belonging with a group again. Skyline assumes that any number of people from all nearby tribes could have been tempted by the promises of the Eclipse, regardless of their background.
It’s also just as plausible that characters were “in” the Eclipse without understanding much about what the group was actually doing. For example, people may have been brought in to help with excavation, or supply food for the other members, or guard the camps, and so on. Not all former-Eclipse have to be evil or actively trying to bring about the end times, even if that’s exactly the assumption others might make.