WARNING: The following contains spoilers for Mark of the Devil, now streaming on Netflix.
Netflix's Mexican horror, Mark of the Devil, has a frenetic final act whereby two priests, Karl and Tomás try to save a teenage girl, Camila, after possession by a demon. Camila and her sister, Fernanda, messed with a Necronomicon-esque book their mom, Cecilia, brought home to study, and the priests realize Camila is now the host for Cthulhu's daughter, Cthylla.
However, Karl has a secret of his own as he's the vessel for Cthulhu's son, Ythogtha, except he's rebelling against his dad and wants to banish his sibling and other minions back to hell. This leads to a brutal showdown where Cthylla shows she's no pushover when it comes to demonic warfare.
THE DEMONIC SPLIT
When Camila becomes fully possessed as the priests attempt the exorcism, she kills Cecilia. This leads to an enraged Karl going after Camila with a bit more intensity. However, Camila has a back-up plan as Cthylla splits her satanic essence in two and possesses Fernanda as well. Fernanda smashes her dad, Luis Miranda's head, into the door as he tries to escape, killing him also.
The sisters double team Karl as Tomás has been left worse for the wear. Luckily, Tomás manages to knock Camila out with a vase, but as he flees, Cthylla seemingly reunites her split halves in Fernanda's body. She goes after him but just as she's about to choke him to death, Karl intervenes and reveals the method by which he's been sending his father's followers back to hell all this time.
THE GATEWAY TO HELL
Karl was performing rituals throughout the film but isn't clear exactly what happened. He restrains Fernanda and sucks the black soul of Cthylla out through his mouth, but he now reveals a spiral tattoo on his torso as he rips his clothes off. It glows red, revealing it's the gateway he's been using to send demons back to hell.
However, with Cthylla fighting inside him, Karl/Ythogtha knows there's one true way to ensure she doesn't escape and it's to kill him. Tomás reluctantly puts a bullet through Karl's head, killing his comrade -- someone he nurtured since finding him as a misguided kid with Ythogtha inside. He drops to the floor dead, but thankfully, the girls are safe. The priests' mission comes to an end but it's at a great price as Tomás mourns his brother-in-arms, someone who literally turned on hell's army itself.
THE AFTERMATH
Tomás is left to secure the book, and he eventually cleans up his act and his church as well. He takes Karl's body to a cave, reciting scripture and paying homage to Jesus' followers who wrapped him in white cloth in a tomb after his crucifixion. Tomás leaves Karl's body there, hoping his friend finds peace and maybe even heaven in the afterlife. Fernanda and Camila move out as they can't bear to live in that house anymore knowing they killed their parents. Interestingly, Fernanda gives Camila a locket, one which Camila lost in the film. When she was initially possessed, the necklace got stuck in Camila's skin, though, so clearly Cthylla had a liking for it.
Some believe Cthylla could have imbued part of her essence in the trinket, and seeing as Camila was never exorcised, it's possible she still has remnants of Cthylla in her. Coupling this with the necklace, suspicions are Cthylla might not be done with the sisters, and this could reactivate the spirit inside. Notably, as the mid-credits sequence also hints, this Cthulhu family drama is far from over.
Now streaming on Netflix, Mark of the Devil is directed by Diego Cohen. It stars Eduardo Noriega, Eivaut Rischen, Arantza Ruiz, Nicolasa Ortíz Monasterio, Lumi Cavazos and Omar Fierro.