On From Dusk Till Dawn Season 3 Episode 6, Richie wakes up in an abandoned asylum, the same one that Amaru was in at the start of the season. After getting out of his straitjacket, he manages to find a phone to call Seth.
Seth and Burt are instantly aware that it's a trap, but it's Richie, so it doesn't matter. The team gears up and rides out. Shortly after reuniting with Richie, things start to go sideways.
Richie puts the whole place into lockdown, and then he starts picking off everyone one at a time. Amaru got inside Richie's head and awakened his shadow self - the Xibalban version of himself that's loyal to only her.
Amaru wants Richie to kill his friends off one by one, so he can gain admittance into Xibalba. Richie's got a better plan. Everyone will still die, but Seth is going to be the one to kill them because Richie isn't leaving with Seth.
Seth isn't too thrilled about this arrangement. He pleads with Richie to stop, but it doesn't work. Thankfully, Burt has a back up plan. He had read that there's a purification ritual to cleanse someone of Amaru's hold. All it takes is setting the inflicted person on fire.
Seth sets Richie on fire, and thankfully, it works. Unfortunately, Seth didn't save his brother soon enough. As Seth and Richie are fighting, Richie uses his power on Freddie to get him to kill Ximena.