On Dusk Till Dawn Season 3 Episode 3, Freddie comes home to witness a culebra stake himself in Freddie's living room. This, naturally, prompts Freddie to search for clues to help identify this culebra. All roads lead back to Sex Machine, unfortunately.
Sex Machine is currently teaching at a community college, and on the side he turns his students into culebras and has ragers at his house. When Freddie confronts him about the dead culebra, Sex Machine figures out that Hector (the dead culebra), had his venom glands removed. This tidbit plus Freddie name dropping Xibalba leads Sex Machine to the culprit. A ranch hand from Xibalba is castrating culebras by removing their venom glands in order to keep the Xibalban hierarchy alive.
Meanwhile, Richie and Seth manage to find an ancient hunter who has kicked Xibalban ass before. his name is Burt, and he runs as a gas station, as you do when you retire. Burt is able to clue the brothers in on what happened to Kate: she has been taken over by Amaru, the Queen of Xibalba, and she plans to open the door to hell.
Thanks to Burt, Freddie, and Sex Machine, the ranch hand is defeated, but Amaru rescues his slaves before Freddie and company can take care of them. Out in the desert, Amaru walks up to the door of hell.