The power to be eternal and indestructible. An advanced version of Immortality.
Banned from Death/Dying
The user possesses absolute immortality, is unable to die, age, get sick, or be permanently wounded, is absolutely self-sustained, and as the user's mind and soul are as immortal as their biological bodies, they are immune to mental/spiritual damage. Any injuries the user suffers immediately heal, even if the user's body is disintegrated or blown up or completely deleted, even if the user is completely destroyed to the sub-atomic level, they will still return to life. Alternatively, some users are absolutely immune to all harm, nor can they die of any natural causes.
The user's existence and soul become completely independent of even the concept of reality, making them not bound to the subjects of life, death and manipulations. Also, the power cannot be removed, rendered truly perfect and absolute.
Ladylee Tangleroad (A Certain Magical Index) ate Ambrosia, which twisted her cycle of life force, granting her complete immortality that she could not get rid of.
Sōsuke Aizen (Bleach) attained immortality and indestructibility after fusing with the Hōgyoku, which kept regenerating him no matter the damage, resulting in the necessity to seal him away deep in the underground prison.
Lobo (DC Comics) has caused too much damage in both heaven and hell that he has been permanently banned from the afterlife, rendering him completely unable to die, even when reduced to a puddle of blood that he regenerated out of.
Garlic Jr. (Dragon Ball) succeeded in making a wish for immortality with the Dragon Balls, allowing him to regenerate from all bodily damage and never age nor die.
Porky Minch (EarthBound) abused time travel so much that he trapped himself in the current timeline, resulting his inability to age further nor die in anyway. He lived to the end of the world, and is still alive.
Bolt Crank (Eat-Man) is so immortal that not even Death could affect him in any way, despite Death physically manifested in an attempt to end his unnatural agelessness.
Kagerō (Flame of Recca) used the forbidden time-portal spell Jikuryūri (Time-Space Separation), which traps her outside of space-time, cursing her to become immortal.
Sun Wukong (Journey to the West) become unable to die or be harmed in any way after eating both the food of the heavens and erasing his name off death's register.
Raziel (Legacy of Kain) is the agent of the Elder God, and is beyond death.
Deadpool (Marvel Comics) has one of the most powerful healing factors in the world, and Thanos even cursed him to live for all eternity, banning him from Death's realm.
Thanos (Marvel Comics) is an Eternal who was banned by Death itself from entering its realm, and thus is unable to die in any way.
SCP-682 (SCP Foundation) possesses regenerative properties so advanced that every attempt the SCP Foundation has made at ending his life has failed miserably.
Death (Supernatural) is, as his name suggests, the personification of death, and is absolutely immortal as long as death exists, which since mortals eventually die, Death's aspect may well remain forever.
Because Timmy Taco wished that he and Sally Sandwich (Sweet Story) would be "married forever", it made it so that neither could ever die - including immunity to the effects of starvation or thirst - the wish making sure that their marriage does not end through death due [they] part.