Immortals are beings who are immune to death through any conventional means, which is what separates them from regular mortals. This category includes the Egyptian, Norse, and Greco-Roman gods, the Titans, the Giants, monsters, and some demigods who have accepted immortality as well. There have also been instances where a mortal or demigod has been gifted with immortality by the gods, usually as either a reward for great heroism, e.g. to Hercules and Percy Jackson, or so that they may be united in marriage with a god, such as in the cases of Psyche and Ariadne. The Hunters of Artemis are gifted with immortality as long as they keep their vow of maidenhood and do not fall in battle. The einherjar of Odin are also granted with immortality (within Valhalla) until Ragnarök. Nymphs and satyrs can be considered immortal in the sense that they will never die of age. They are, however, subject to death by other means.
Immortals are immune to death in any form. They do not age, nor can they die from poison or disease. They are immune to all weapons that mortals use, as they are not forged from magical metals such as celestial bronze, Stygian iron, Imperial gold, or bone steel (Norse gods only). 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 normal mortals, and they are typically hidden from human view by the Mist. However, immortals tend to show some predictability, and most tend to be very prideful, arrogant, not willing to accept help from lesser beings even to the point of fading. Also immortals tend to follow "patterns" as the Second Olympian War and the Giant War are both just history repeating itself.
The immortality of Egyptian gods has it limits:
- Killing and Duat Banishment: Egyptian gods can be banished back deep into the Duat either by being killed, which is possible, or by ensnaring them through very powerful and effective magical items such as the Seven Ribbons of Hathor. A magician can also banish them through certain spells that require their secret names or through execration. However, they will eventually return, as killing a god just kills their mortal body and drains their power, and the banishment will eventually end.
- Shadow Execration: Gods can actually be permanently destroyed if a magician casts an execration spell on their sheut. According to Carter Kane, this is because if a person casts no shadow at all, their existence is meaningless. Destroying a sheut cuts all ties to the mortal world that a being has as well as the fact that the sheut is an important part of the soul. This is used to destroy Apophis, though it takes the combined power of Carter and Sadie Kane as well as Horus and Isis to do so without Carter and Sadie being destroyed in the process. Gods keep knowledge of this secret so it can't be used against them, as a magician threatening to execrate their sheut is effective blackmail against them.
- Dependence on Ra's Life Force: If Ra the sun god is missing, the gods start to lose power slowly. Worse, if Ra is somehow destroyed (via shadow execration, for instance) all the other gods are destroyed as well.
- Energy Depletion: Bast mentioned that it is possible for gods to cease to exist if they expend too much energy without stopping to rest. However, this is clearly quite a lot, as Bast and Apophis dueled continuously for thousands of years nonstop but did not cease to exist.
- Scattering: if the body of an immortal being is damaged enough, they may be weakened into a state where they can no longer reform (for many centuries, at the very least), even to the point of not being able to gather a complete consciousness. Ouranos, Kronos, and Gaea are the only known immortals to have succumbed to this fate. The extremely few weapons powerful enough to destroy the bodies of immortals in such a way are Kronos' scythe, Zeus' master bolt, and Poseidon's trident.
- Secure Imprisonment: Immortals can also be sealed away in extremely secure prisons, such as the bronze fortress in the depths of Tartarus, which was the fate of most of the Titans. Atlas was notably imprisoned on Mount Othrys, unable to leave while the incredible weight of his celestial burden rested on his shoulders.
- Seat of Power Destruction: Immortals are always severely weakened when their thrones (representing their seats of power) are destroyed. Zeus ultimately used this method to prevail against the Titans at the end of the First Titanomachy, and Kronos was almost successful in using the same method against the Olympians at the end of the Second Titanomachy. The Giant King Porphyrion theorized that a similar yet even more severe weakening of the Olympians could be achieved by destroying the Athenian Parthenon, as it was located at the gods' "roots" as the original Mount Olympus was located in Greece.
- Fading: Immortals can also "fade" away into nothingness, ceasing to be, should they lose the will to live or be forgotten, which has happened to several immortal beings, such as Helios, Pan, Selene, the two brothers of Briares, and the two sisters of Medusa. They also represent the state of their realm as shown by Poseidon.
- Tartarus Essence Absorption: The infernal Lord of the Pit, Tartarus, has the ability to suck beings into the swirling whirlpool of darkness on his face, which he did to the immortal essence of Hyperion and Krios. In that sense, Tartarus can technically "kill" gods, Titans, and giants by absorbing their immortal essence into his armor for very lengthy periods of time.
- Void of Chaos: Chaos is believed to be one of the few things able to truly kill an immortal (possibly by permanently absorbing their divine essence with it being impossible for it to ever escape), as Zeus held Hera over Chaos after her rebellion, threatening to drop her into the void.
The immortality of the Norse gods has its limits as well:
- Apple-Induced Youth: The Norse gods will rapidly grow old and weaken when they're unable to eat Idunn's apples of youth on a regular basis, as shown when she was kidnapped by the giant Utgard-Loki on one occasion.
- Mortality: The Norse gods, while extremely difficult to kill (Mimir survived being decapitated, for instance), are not completely immortal (unlike the Greco-Roman gods), as most of them are destined to die during Ragnarök, the Twilight of the Norse gods. Beings or weapons with the power to kill Norse gods include the following:
- Other gods: Loki and Heimdall are destined to kill each other.
- Surt: The Lord of Muspellheim, who is destined to use Sumarbrander to kill Frey.
- Fenris Wolf: The ferocious wolf is destined to kill Odin.
- Garm: The blood-stained guard dog of Hel is destined to kill Tyr.
- Jormungand: The World Serpent is destined to kill Thor.
- Mistletoe: The only weakness of the otherwise invulnerable Balder, yet one that pertains to him only.
- Mjølnir: the hammer of Thor, with which he has has threatened to kill Loki several times.
In both ancient Greek mythology and in the series, heroes who have been distinguished for remarkable talent have been given the choice to become immortal. However, it is possible for the chosen hero to reject the gift.
Hercules was the son of Zeus. He was a great hero known for the Twelve Labors Of Hercules. The gods recognized that and he was made immortal upon his death.
While never immortal, Percy Jackson was given the choice of immortality after he defeated the evil Titan lord Kronos. Percy refused, deciding he would rather be with Annabeth Chase than live forever. This caused a moment of confusion for the gods, since most mortals would accept the gift of immortality. He did, though, consider it for a moment, saying he would never age, never die, and could avoid what the Fates showed him: his whole life flashing before him until his death. In the end, he refused the gift and instead made a request to the gods to make lives better for demigods and lesser gods (and also peaceful Titans).
The Hunters of Artemis are Artemis's loyal maidens and sisters-in-arms. They have sworn an oath to her, vowing to remain a virgin forever. With this, they became immortal. They cannot die from disease or age, but they can die in battle. If they break their vow to Artemis by falling in love, they are once again rendered mortal. The Hunters also gain speed, strength, and become experts with the bow and arrow, as Artemis, along with her brother Apollo, are the twin archers. Bianca Di Angelo said she felt different after taking her oath and becoming immortal. Thalia Grace claimed she must join the Hunt as she never felt peace since Half-Blood Hill, and she felt as if she now had a family with the Hunters. This might be because time moved slower when she was a pine tree, and seemed to speed up as a human, and slowed down again when she became immortal.
The giant children of Gaea and Tartarus are immortal and will heal from mortal wounds at incredible speeds, unless killed by the combined efforts of a demigod and a Greco-Roman god. It is implied, yet never confirmed, that a giant can also be killed by a demigod working together with a Titan instead.
The only one of the giants with a different form of immortality is Alcyoneus, as long as he remains within his native territory (Alaska in The Son of Neptune), and will regenerate after mortal wounds at incredible rates, without or without godly assistance. However, when taken out of that territory, the giant can be killed by any means.
Odin's einherjar cannot be killed while they remain within Valhalla, and will simply keep resurrecting from mortal wounds every time. They do not, however, have this ability outside of the hotel while they visit any of the Nine Worlds, where they are rendered completely mortal.
While monsters can age, most monsters can't die as mortals do and will eventually come back. Whenever a monster is killed, their essence will return to Tartarus and reform, allowing them to return to the mortal world. This is true for almost all monsters, giving them some form of immortality as they can never truly die. However, the time it takes to reform can take anywhere from a few days to a few centuries to a few millennia.
There are exceptions, however, as not all monsters die in the same way. Satyrs for example will be reincarnated into something in nature when they die (such as a laurel). Also, dryads and water nymphs can live forever, as long as the thing their life-force is tied to isn't destroyed (plant life and bodies of water respectively). If their life force is destroyed, they will die as well.
- Main article: Fading
Fading is the only true way for most immortals to die. In the case of a god, if their realm is destroyed or not enough people believe in them, like in the case of Pan, they will fade from existence. In the case of Briares' brothers, they had faded because no one believed in them, but Percy suggested that they also stopped believing in themselves. The same can happen to monsters if not enough people remember them, but Gaea managed to find a way to reverse this somehow, as she brought back Medusa's sisters, who Medusa had claimed faded in The Lightning Thief.
- When Grover Underwood explains to Percy about what it means to be immortal in The Lightning Thief, Percy thinks it sounded like a pretty good deal. However when he was offered to become immortal in The Last Olympian, he refused in favor of another gift.
- Mars seems to believe that mortals have it easy because they can die, and said something along the lines of it in the Son of Neptune while talking to Frank Zhang.