|“||Percy, even immortality has limits. Sometimes…sometimes monsters get forgotten and they lose their will to stay immortal.||”|
Fading is caused, at least partially by the diminishing of a god realm or people not believing in them. Gods that are not well remembered or were never worshiped can also fade. Gods seem to be able to resist fading on some level.
If a monster or god loses their will to remain immortal or lose faith in themselves, it can have great influence on whether or not they fade. Pan for example sent out a message to tell the world he was dead, in hopes that he would be able to fade. However, the satyrs believed in Pan so strongly that he was kept from completely fading, meaning that beings are not always able to fade and can't choose when it will happen. It is unknown if a god must consciously give in to the process in order to fade or if it will inevitably overtake them.
While fading is considered to be the only way for an immortal being like a god or monster to die, Gaea was shown to be able to revive at least Stheno and Euryale, hinting that the process can be reversed for monsters. Chiron also mentioned that a demigod once Googled the Gorgons, resulting in them appearing even if they had already faded (with the exception of Medusa).
When talking with Medusa, she says that she used to have two sisters, but they are no longer with her. This is because while Medusa is very well known, her sisters weren't as 'popular' and they ended up fading because people didn't pay much attention to them.
While Apollo drove a group of demigods and Hunters to Camp Half-Blood, he was asked why sometimes he drove the Sun Chariot and other times it was Helios. Apollo explained that the Romans started "downsizing" the number of gods, resulting in Helios' and Selene's roles being given to Apollo and Artemis and they eventually faded.Demigod children of Hades can detect the fading of gods, or perhaps just when a dead or semi-dead god is near, as seen by Nico di Angelo when he reveals that Pan is indeed fading, that what they see is more like a memory that can't pass on.
Pan is the only god that Percy Jackson has seen fading during the entire series. His fading was caused by the rapid diminishing of his realm, as there were almost no wild places left on earth. He was not able to completely pass on ,however, as the satyrs never believed that he had died and their belief kept him alive for centuries. Pan would spend these centuries in a state of hibernation, waking up for small periods of time to conserve his power. Just before he faded, he passed his power on to Grover Underwood and friends, giving the implication that fading gods can entrust a portion or all of their powers to another being. As Percy watched him fade, he remembered what Apollo had told him about the gods Helios and Selene, as well as what Medusa told him about Stheno and Euryale fading.
Stheno and Euryale, Medusa's sisters who faded, were revived by Gaia and attacked Percy. It is unknown how Gaea resurrected them as they had faded.
During Magnus' first breakfast in Valhalla, his hall mates tell him that he should not close himself off and to occupy his time. They tell him they if he dies not, he will fade.
Thomas Jefferson Jr. tells Magnus that one of the soldiers he killed just before he died came to Valhalla as well, but faded by the time the son of Frey arrived.