The Eidolon Wyrm is a colossal beast that lives in the depths of the abyss, near the hadopelagic zone. While one might think these giant serpents are just part of nature, it is revealed that they have a long, tragic backstory. Approximately 500 years ago, the lunatic cult forced its people to construct a massive dungeon, expanding underground with hundreds of tunnels and dark rooms. This structure was used to keep Cthulhu at bay, from destroying and spreading evil across Terraria.