Immortals are immune to death in any conventional form. They do not age, nor can they die from poison or disease. They are immune to all weapons not forged from magical metals such as celestial bronze or stygian iron. If their bodies are somehow harmed or destroyed they will eventually reform, though the length of this process can vary from days to centuries. Almost all immortals have supernatural abilities far surpassing those of humanity, and they are typically hidden from human view by the Mist.
Immortality does, however, have its limits. If the body of an immortal being is damaged enough they may be weakened to a point where they can no longer reform, even to the point of not being able to gather a complete consciousness. They can also be sealed away in prisons such as the pit of Tartarus. They can even "fade" away into nothingness, ceasing to be should they lose the will to live, which has happened to several immortal beings such as Helios, Pan, and the brothers of Briares.