Nightmare Lights are light sources found within the Ruins. They cannot be opened or destroyed with a Hammer. Instead, the amount of light they emit depends on the current state of the Nightmare Cycle that the Ruins are in; the more extreme the magical state, the more the Nightmare Lights open up.
Nightmare Lights drain the player's Sanity when they are open and if the player is near. The actual magnitude of the drain depends on how much light is coming from the Nightmare Lights (how "open" they are). During the Warning phase they will drain 600 sanity/min, while in the Nightmare phase they will drain 900.
Shadow Creatures will appear around the lights, even if the player is not insane. The Shadow Creatures only appear during the Nightmare phase and are hostile to the player. Once the Nightmare Cycle is completely over, the creatures will die, dropping Nightmare Fuel in the process.
Nightmare Fissures are objects that appear in the Ruins and rarely in Caves, more specifically, in the Stalagmite Biome. They act identical to Nightmare Lights. Depending on the phase of the Nightmare Cycle, they emit more or less light - the greater the light radius, the higher the chance of spawning Shadow Creatures.
Each fissure can only spawn one creature, either a Terrorbeak or Crawling Horror. If the player has been detected by either one of these mobs, all other Terrorbeaks or Crawling Horrors in the vicinity will become solid and will be able to attack the player in a group. Once the fissure is closed, the first spawned Shadow Creature will automatically die, and the other ones will become neutral, assuming the player has enough Sanity.
Nightmare Fissures will reappear at the same location after a period of time. It is advisable to put a Sign or other item with a Map icon near these fissures to mark their locations, since these are the best places to find Nightmare Fuel.
Ancient Obelisks are naturally spawning structures exclusive to Don't Starve Together, introduced in A New Reign. They replace Nightmare Lights in the Atrium Biome in the Ruins. Instead of activating during specific Nightmare phases, Ancient Obelisks are activated by inserting the Ancient Key in the Gateway.
Unlike Nightmare Light, the Obelisk's insanity aura is not dependent on it being active. At 800 sanity drained per minute, its effect is similar to that of a fully-open nightmare light, and players should avoid standing close to one, lest they have means to restore the sanity and get past the Obelisk blockades.
- Nightmare Fissures were introduced in the Six Feet Under update.
- Sometimes, Nightmare Lights and Fissures will open during the Calm State. During this time, they will emit blue light instead of red or pink light.
- Ancient Obelisks were added in the Heart of the Ruins update.
- Maxwell's examination quote for the ancient obelisk implies that he once knew the ancient language.