Immortality in fiction

Immortal beings and species abound in fiction, especially fantasy fiction, and the meaning of "immortal" tends to vary.

Some fictional and mythological beings are completely immortal (or very nearly so) in that they are immune to death by injury, disease and age. Examples include various types of gods and the evil Cthulhu. Sometimes such powerful immortals can only be killed by each other, as is the case with the Q from the Star Trek series. Even if something can't be killed, a common plot device involves putting an immortal being into a slumber or limbo, as is done with Sauron in J.R.R Tolkien's The Lord of the Rings and the Dreaming God of Pathways Into Darkness. Storytellers often make it a point to give weaknesses to even the most indestructible of beings. For instance, the Greek hero Achilles was supposed to be invincible, yet his enemies were able to use his infamous weakness to slay him. 

Many fictitious species are said to be immortal if they cannot die of old age, even though they can be killed through other means, such as injury (known in some role playing games as "limited immortality"). Modern fantasy elves are an example of such a species, though in some cases they have a long but finite lifespan. Other creatures, such as vampires and the immortals in the film Highlander, can only die from specific injuries. The list of such injuries for vampires varies somewhat on the particular work or inspiring mythos; for example a stake through the heart may be anywhere from inconsequential to incapacitating to instantly fatal. 

A fictional creature might also be considered immortal if its life span is just unimaginably long. For instance, the dragons in some fantasy works can eventually die of old age, but often their lifespan is measured in the thousands of years (or perhaps longer), so they're considered immortal by shorter lived species (such as humans). Elves and other fairies in some works also fall into this category. Beings like this often remind humans of their greatest weaknesses by uttering the derisive cliché "mere mortals". 

Immortality can be used as a prize, something to be earned by great achievement. Legendary heroes, great magicians and wise elders sometimes rise to the ranks of immortality in fiction and mythology. It can be the reward at the end of a great quest, such as the quest for the Holy Grail or the ancient Epic of Gilgamesh. When immortality is something that can be bought, works of fiction will often make judgements regarding the high price that must be paid. Immortality is often the desire of evil characters as well. If immortality is something that can be earned, then it can also be taken away, much to the dismay of many an immortal villain. 

Since immortality is seen as a desire of humanity, themes involving immortality often explore the disadvantages as well as the advantages of such a trait. Sometimes immortality is used as a punishment, or a curse that might be intended to teach a lesson. It is not uncommon to find immortal characters yearning for death. A similar, though somewhat different theme, concerned Elves and Men in Middle Earth. While the immortality of Elves was not explicitly a curse, the mortality of humans was viewed as a gift, albeit one that the immortal beings, and often even the humans, didn't understand. 

In some parts of popular culture, immortality is not all that it is made out to be, possibly causing insanity and/or significant emotional pain. Much of the time, these things only happen to mortals who gain immortality. Beings born with immortality (such as deities, demigods and races with "limited immortality") are usually quite adjusted to their long lives, though some may feel sorrow at the passing of mortal friends, but they still continue on. Some Immortals (such as certain deities, demigods, and intelligent undead) may also watch over mortal relations (either related to or descended from them), occasionally offering help when needed.