Mortality Ensues

(Mortality, not morality)

An immortal character becomes mortal, usually via external means.

Overlaps with Nigh Invulnerability when the immortality is due to invulnerability and no longer having the latter means no longer having the former. Commonly done to Big Bads so that they can be killed. Sometimes such a way is sought by the immortals themselves. At other times the way is avoided at all costs.

Examples of Mortality Ensues include:

Film

In The Mummy 1999, it turns out that the companion book to the Book of the Dead doesn't kill the Big Bad when read from. What it does is remove his immortality, allowing him to be killed like a mortal. Imhotep, however, didn't know this and simply walks into a sword, assuming he'll be fine.
Pirates of the Caribbean has this happen to Barbossa at the worst possible time; Will drops the final gold piece into the cursed treasure chest just after Sparrow blows a fatal hole in Barbossas chest with a pistol, removing the curse and causing Barbossa to bleed out.
Part of The Prize in Highlander is the option to live as a normal human, which Connor states is his intention at the end of the original film.
In Disney's |Hercules, Hercules gives up his demigod status to save his Love Interest. Although not technically immortality, it comes close.

And Another Thing: Wowbagger the Infinitly Prolonged wants to die, but can't because he's been made immortal against his will. At the end he become de-immortalized, so he will eventually die. Close Enough.
The plot of Harry Potter and the Deathly Hallows was Harry and company searching for and destroying the Horcruxes in order to remove Voldemort's immortality.
In J. R. R. Tolkien's Middle-earth, the members of the Half-elven family descended from Luthien get the cosmic choice to which kind they want to belong, leading to (im)mortality and a different fate after death. Arwen from The Lord of the Rings, and before her her uncle Elros, choose humanity and mortality. Their ancestor Luthien was Elven, but got a once-only cosmic exception to have a mortal fate to be with her human lover Beren, before and after death.

7 of 9 encountered this trope in Star Trek: Voyager - she hurt her hand while working and was upset with the fact the supressing the Borg nanotechnology removed her Healing Factor.
- - Q in a Star Trek: The Next Generation episode had his omnipotence taken away as a punishment, thus becoming mortal.
  - Who then turns around and does this to his son q (yes, lower-case) in Star Trek: Voyager. Unlike the above example, it's less of a punishment and more of a disguised lesson.
On Angel according to the Shanshu Prophesy, the Vampire with a Soul will eventually be rewarded by dying - which is to say, he'll be rewarded by becoming human, and thus being allowed to die as a human.
Nathan from Misfits sells his immortality on the black market.
The clone of David Tennant's Doctor, grown from his severed hand, was cleansed of Time Lord DNA and will have a normal human lifespan, living with Rose in a parallel universe.
The final episode of Lost implies that Richard Alpert has lost his immortality, which he considers a very good thing.
In Torchwood: Miracle Day, Captain Jack Harkness loses his immortality.
- At the end of the series, this happens to everyone, except Jack and Rex.

This trope was the culmination of a centuries-long Gambit Roulette against Fu Leng in the official storyline of the Legend of the Five Rings RPG/CCG setting.
This is a goal of the main character of Planescape: Torment.

In his best ending in Soul Calibur 3, Zasalamel succeeds in becoming mortal and spends the rest of his life as a scholar, chronicling everything he experienced.
In Final Fantasy III, the great sage Noah awards each of his three students with a gift. Doga is bestowed with great magical power while Unei was given control over the world of dreams while she slept, and both go on to become renowned sages in their own right. Xande? He receives the "gift" of mortality. He doesn't take well to this snubbing.
The Big Bad of Final Fantasy V, as well as the previous wielder or the Void, both lost their immortality when they acquired this power.
In the finale of World of Warcraft Cataclysm the Dragon Aspects lose their immortality (and possibly more of their powers) after focusing their power through the Dragon Soul to slay Deathwing for good.

This is basically what usually happens to liches once their phylactery is destroyed: they don't immediately die from that, they just become killable.