"Only one can live ... torment to all ... witness for eternity ... torment for eternity," Paulina shakily translates from the messages scrawled on the cavernous walls. The characters revisit this message throughout the film, dismissing it as ancient folklore. However, this was the curse they'd soon fall victim to.
Dave recognizes the accompanying drawings from a book he read on Chamorro folklore. He explains that the first Bojobo dolls held the spirits of an ancient chief and his bride who shared an eternal bond. In death, they were separated and the curse was formed. "The chiefs' suffering for his lost love would be equally eternal and that torment would be felt ..." he says.
The group focuses on the "only one can live" statement, which prompts Alan to attack Tyler, Pepe, and Paulina. He believes that being the last man standing will free him from the curse and the bunker. It's not until Tyler, with his last breath, laughs and says, "If you live, you lose. You lose worst of all."
Alan then realizes that they were never going to escape the bunker. They cannot die unless someone witnesses their passing and replaces them as the eternal witness to the horrors of Gehenna, meaning the sole survivor cannot die unless someone new enters the bunker.
The end of the movie also reveals that the fluidity of time forces Alan to keep repeating the incident without being able to prevent it — he was the ominous skeletal man all along. Unless, of course, the post-credits scene has anything to do with what happens next.
