This tag is used if the novel features a character class/group of people/race that are called 'Immortals'. Most importantly, the 'Immortal' term is defined within the context of the novel's world setting, and not ours (for example, the immortals in 'I Shall Seal the Heavens' did not have immortality; they simply has long lifespan and will still die of old age).
Humans are clever in tens of thousands of ways, Gu are the true refined essences of Heaven and Earth. The Three Temples are unrighteous, the demon is reborn. Former days are but an old dream, an identical name is made anew. A story of a time traveler who keeps on being reborn. A unique world that grows, cultivates, and uses Gu. The Spring and Autumn Cicada, the Venomous Moonlight Gu, the Wine Insect, All-Encompassing Golden Light Insect, Slender Black Hair Gu, Gu of Hope… And a great demon of the world that does exactly as his heart pleases! — *Gu (蛊) referred to poison in various cultures in South China while in folklore Gu spirits could transform into all kinds of species.