Everworld is the place where the ancient gods of earth went eons ago, taking their followers with them, but leaving most of humanity behind. As Christopher explains, 'Somehow, someway, for some reason, the old gods of Earth decided to abandon the real world. We didn't know why. Just knew that the gods of the Norsemen and the gods of the Greeks and the gods of the Aztecs and the Inca and the Egyptians and all the endless panoply of immortals, all decided they'd had enough of the real world. Our world.' A terrifying being that consumes gods is headed for Everworld.
One problem is that the gods are thinking of returning to earth, and the Norse god Loki is trying to open a way back to earth for himself and the other gods in order to escape. Once back on earth, they would reassert their rule over humanity. The blood sacrifices to feed Huitzilopoctli are merely one example of the horrors that would fall upon human beings. Somewhat nicer gods than Loki (such as Thor and Odin) have disappeared because of Loki's treachery, so it seems that Loki is not necessarily intent on taking all the gods back to earth; he may wish to take only the evil ones. (He will claim otherwise in Enter the Enchanted, but can his word be trusted? ) Christopher, David, April, and Jail were drawn into Everworld when a giant wolf kidnapped Senna, who is, unbeknownst to them, a witch. They are all pulled into a vortex that seems to turn them inside out at first.
It takes them much of Land of Loss to figure out what happened to them. Early on, they suspect that Senna exerts some sort of mind control over David, and perhaps Christopher, who was her boyfriend before David became her boyfriend. By the novel's end, they have put together some of the pieces of the puzzle that is Senna. She is a teenager on earth, but, in Everworld, she is the witch who is the key to opening the passage to earth for Loki, Huitzilopoctli, and others who live in Everworld.