The Real sun: eaten 700 years ago by the Twelve-Winged Devourer
seven gigantic orbital rings (the ring station in that generate artificial sunlight and hold back the eternal night. Each ring is ruled by a "Stellar Lord" who is basically a demi-god.
Ground level: perpetual twilight, toxic red mist, mutated monsters, fallen cities swallowed by living darkness
The Devourer is not dead — it is sleeping inside the hollowed-out core of the sun, slowly digesting it. Every 77 years it wakes up a little and takes one more bite. The next bite is in 3 years. When the twelfth bite is taken, the world ends for good.
Power System (unique, not generic)The "Stigma System"
Every human is born with a Stigma — a black mark somewhere on the body that is literally a fragment of the Devourer's dream. The more you use your Stigma's power, the more the Devourer wakes up and the closer you slide toward becoming a "Hollow Saint" (insane tentacle monster).
Si-Hyun's Stigma is unique: it covers his heart and is shaped like a closed eye. When he opens it, he can use the Devourer's authority directly — but every use literally shortens the time until the final bite.