This is a relatively slow and subtle movie, with a lot of quiet revelation and no real scares. But it has a lot of horror, of the existential dread variety. Though there are little hints of something Lovecraftian going on (Lovecraft gets a name drop on the brothers’ first day at camp, there’s some talk of impossible and imperceptible colors, and there are hints of some unfeeling and inhuman power lurking just beyond perception), “The Endless” is much quieter than explicitly Lovecraftian movies like “Re-Animator” or “Alien.” This is the philosophical, nihilistic side of Lovecraft; deeply unsettling, skillfully shot, and with characters who are both believable and bizarre.