Immortality

Immortality (or eternal life) is the concept of living in a physical form for an infinite or inconceivably vast length of time. Immortality is one of the known limits to magic; it is impossible to make oneself immortal unless one makes use of a mystical object of great power to sustain life (such as the Philosopher's Stone created by Nicolas Flamel or a Horcrux, the latter having been used by Lord Voldemort). If one were to possess the three Deathly Hallows, it is fabled that they would possess the tools to become the "master of death". However, being a true "master of death" is to be willing to accept that Death is inevitable.

Philosopher's Stone
The Philosopher's Stone, a Stone created by famous alchemist Nicolas Flamel, is able to produce the Elixir of Life, one of the known means of immortality. In 1991 and 1992, a weakened Lord Voldemort tried to gain possession of the Stone so he could rise again after his downfall ten years prior. The Stone was then destroyed by Albus Dumbledore and Nicolas Flamel himself in order to prevent this from happening again. With the destruction of the Philosopher's Stone, all individuals who were immortal because they drank the Elixir of Life (like Flamel and his wife) died after the supply of Elixir ran out.

The Elixir does not truly grant immortality, as it only extends the drinker's lifespan, as opposed to rendering them invulnerable to damage. Thus, it is possible for them to die even while drinking the potion.

Horcruxes


A Horcrux is an object chose to the purpose of being a receptacle of one's part of his soul, split by doing the most inhumane action: murder. If all the Horcruxes (and by extension the wizard's soul) are intact, the wizard is considered immortal.

Lord Voldemort split his soul seven times in order to maintain his status of immortal being, and kept his Horcurxes a secret from absolutely everyone to protect his own life. He had split his soul that many times in the likely belief that seven is a powerful and magical number. The seven Horcruxes were all items owned by reputable people that played an important- or scarring- role in his life, including the Four Founders. His best plans were, however, beaten due to his arrogance, when Albus Dumbledore, Harry Potter, Ron Weasley and Hermione Granger discovered his secret and found each one of his Horcruxes and destroyed them, causing Voldemort to be able to die again.

Phoenixes
Whenever phoenixes die, whether from old age or something like a Killing Curse, they always reborn from their remaining ashes, technically making them immortal.

Unicorn's Blood
Unicorn blood has the gift to save a drinker from death even though they are on the edge of this but if taken, it will lead them to be cursed for all life.