Departure Means Death

Many stories include alternate planes of existence. A magical world, a fairy land, an alternate dimension or an enchanted forest may all have their own inhabitants. Sometimes the residents may be lucky and be able to travel through them safely, if not easily. For others, there's a different problem: They die if they leave their home.

Apparently, there's just something special about them and their home. Maybe there's magic in the air that they need to breathe, or their existence is tied to a certain location or something has been done to the people to limit where they can live. Whatever it may be, if they leave their home they will usually get sick and die.

However, it's not entirely hopeless. There may be a special magical artifact or piece of Phlebotinum that stops the process, counteracts the effects enough to allow continued living or at least slows it down. After all, if two worlds are truly incompatible, it limits the amount of stories you can tell.

Do note that this doesn't really apply to aliens or deep sea denizens or the like. Those are issues of different atmospheres or pressure differences, whereas this is generally more supernatural in nature. This trope is not about humans being unable to live on Venus or polar bears dying in the desert.

Comics

 * Warren Ellis' run on Stormwatch had a superhero whose Weaksauce Weakness was that he couldn't spend more than a few hours outside a city. It made all-hands meetings on the team's space station base awkward.
 * In Thorgal, the three sisters live in a secret valley inside a glacier. The valley is a time anomaly that has allowed them to live for centuries, but they cannot leave for fear of time "catching up to them".

Gamebooks

 * In the Lone Wolf gamebooks and associated novels, the Darklords of Helgedad cannot survive outside the polluted atmosphere of the Darklands without special apparati, which can be magical, but can also include special breathing tanks. Of course, they are attempting to expand the Darklands through conquest.

Film

 * In Field of Dreams, if the any of the ghost ballplayers who play at the field actually leaves it, it turns out that he can no longer return to the mortal world afterwards. What happens if he decides to just not go back to the spirit world isn't explored.
 * In Mighty Morphin Power Rangers: The Movie, Dulcea will return to her thousands-of-years-old true age and die if she leaves... hence her not being able to solve the whole plot without the Rangers' help. Similarly, in the movieverse, Zordon is a guy actually physically in the tube and being sustained by it, not broadcasting from another dimension. The Big Bad breaking the tube means it's a race against time to save him.

Literature

 * In The Talisman, Wolf slowly sickens and loses all liveliness on Earth. Jack is rather frantic to get him home before it's too late.
 * The things that come from the Buick's trunk in From a Buick 8 generally die almost immediately followed by impossibly quick and revolting decay. It's even faster if they come into contact with things. It's speculated that it may be something in the air that does it, but no one's ever quite sure.
 * In Songs Of Earth And Power, creatures from Earth brought into the Realm usually die after a short period of time.
 * If a star leaves Faerie in Stardust, it reverts into a meteorite and thus dies.
 * In The Wheel of Time, after the "Breaking of the World", the Ogier find themselves incapable of being away from their Stedding for any extended period of time; death results if they remain away for too long.
 * In the multi-verse of Phillip Pullman's His Dark Materials, people who live outside their own universe slowly sicken and die. Although it takes a decade or so.
 * In Anne McCaffrey's Crystal Singer trilogy, once you've been infected with the symbiotic organism that allows you to live on the planet where they harvest crystals that allow warp-travel, you can't leave for more than a month or so, or you start going insane and then die.

Live Action TV

 * In Once Upon a Time "bad things" happen to those who try to leave the town of Storybrooke due to the curse the Evil Queen put on it's inhabitants.

Manga and Anime

 * Magic World natives in Mahou Sensei Negima apparently cannot survive on Earth unless they are descended exclusively from Earth natives.

Tabletop RPG

 * Dungeons and Dragons' dryads must stay within a limited radius of their tree. If they pass beyond that radius, they will weaken and die.
 * A Spectral Hunter in Call of Cthulhu must stay within 1 mile of the doll that was used in its creation.

Theater

 * The townsfolk in Brigadoon are unable to cross the borders of their town, lest the entire village disappear.

Video Games

 * in Da Capo II is an example of the artifact type and is unable to leave the island without either Sakura along or Without them a slow fade into nonexistence would presumably occur
 * Angel trainees in Disgaea Hour of Darkness apparently don't have the strength to last long in the Netherworld by themselves, so Flonne carries a pendant that allows her to stay. Naturally, it's stolen at one point, making it an artifact type.
 * The Kokiri in The Legend of Zelda Ocarina of Time can't live outside of the Lost Woods. Ingame rumors suggest they turn into Stalchilds. This is played with when Link leaves the forest for the first time, because he is accepting death (both the Deku Tree's recent death and his own mortality) as part of his Coming of Age Story.
 * Spirits can't survive on Earth for long in Eien no Aselia. There's not enough mana in the air for them. However, on other worlds with mana as thick as Phantasmorgia they seem to do well enough.
 * Elves cannot survive outside the forest or Shinjuku in Big Bang Age. If you keep the elf Sonnet outside of Forestland or Shinjuku for too long, she really will die. It also factors into the character clear of Sparn.
 * Elves cannot survive outside the forest or Shinjuku in Big Bang Age. If you keep the elf Sonnet outside of Forestland or Shinjuku for too long, she really will die. It also factors into the character clear of Sparn.