Nightmare Fuel / Egyptian Mythology

• If you haven't prepared well enough for the journey in life, the Underworld is incredibly unpleasant. After you die, ahead of you is a long journey where you face all manner of evil and frightening monsters and demons that want to eat your soul, as well as unwelcoming geography like lakes of fire. Your only hope to succeed through all of that is having the proper spells and other preparations on your corpse, and more often than not the "surviving" dead become evil spirits.
  • One particularly horrific area of the afterlife is the Burning Pits of Duat, a pre-Hell lake of fire overseen by Am-Heh, the Devourer of Millions, a dog-headed God who likes to eat souls
• Apep (Apophis, as the Greeks called him), as a proto-Lovecraftian concept in the shape of a diabolical snake, is awful enough on its own, but the few records there are of its conception claim it is Ra's umbilical cord. To make matters worse, Apep is completely immortal because he always comes back from every death, no matter how many times Ra destroys him. Humans don't pray to Apep, rather they're worshipping against him, hoping that prayers and rituals will hinder the serpent. In the end, Apep is virtually invincible, and his temporary defeat is merely delaying the inevitable annihilation of all light and life. Sweet dreams!
• Sekhmet's rampage. Ra created Sekhmet to get revenge on a group that conspired against him, but as she killed them, she developed a taste for blood (both metaphorically and literally) and turned into an Ax-Crazy Omnicidal Maniac, gleefully wiping out entire cities and gorging herself on the blood that soaked the ground afterwards. Not even Ra was able to call her off, and she could very well have wiped out all of humanity if she hadn't been magically pacified after getting tricked into drinking so much she passed out.