Planeswalker

In some works of fantasy, a planeswalker is a person with the ability to travel to different planes of existence.

Magic: The Gathering

In the fictional multiverse of the The Gathering collectible card game, a planeswalker is a powerful mage, able to travel across the planes of existence. According to the setting, the potential to become a planeswalker (called the "planeswalker spark"), is innate--very few are born with such a spark, and anyone else who does not possess such a spark cannot possibly become a planeswalker. (Such a spark can, however, be transferred from one being to another, though the process is highly dangerous and potentially fatal.)

The "spark" is not the only requirement for becoming a planeswalker, however--many people who possess the spark never realize their planeswalker potential. A person who possesses the spark must also "ascend", which usually occurs during a time of great stress (most common being a form of horrendous death, e.g. the sylex blast or its after-affects). This ascension, as well as the extraordinary amount of power at their fingertips, drives almost all planeswalkers insane over time. In an attempt to prevent this, most planeswalkers are tutored by older ones. Wilders exist (Ravidel) but are dangerous.

A planeswalker has complete control over his or her physical appearance, and does not have mortal needs, such as the need to eat, drink, sleep, or even breathe (though sleeping helps them retain sanity). Planeswalkers are very difficult to kill and can't die of natural causes. Planeswalkers rarely have relationships with non-planeswalkers due to their near-immortality. Some, however, (such as Urza, the most well-recognized of planeswalkers among Magic) do eat and sleep in order to feel more sane. Their need to do mortal things has become a mental one.

Players are considered planeswalkers in the context of the Magic-card game. Therefore, a planeswalker never gets its own card

Famous planeswalkers from the Magic mythos include:

The Nine Titans

In a last-ditch attempt to stymie the Phyrexian invasion of Dominaria, nine planeswalkers entered the plane of Phyrexia, in gigantic titan engines of Urza's design, to set off bombs to destroy the plane.

Others

the unnamed dragon-planeswalker who created Phyrexia
The Gathering#A
The Gathering#D
The Gathering#F
The Gathering#F
The Gathering)
The Gathering#F
The Gathering#G
The Gathering#J
The Gathering#J
Karn
The Gathering#K
The Gathering#L
The Gathering#L
The Gathering#R
The Gathering#R
The Gathering#S
Serra
Slobad
Teferi
Austin
Aaron
Manoj
Alexandra

In the Kamigawa novels, it is inferred that the Myojins may be some sort of planeswalkers, though not with full power.

Mistaken for planeswalkers

Although they are all thought to be planeswalkers Yawgmoth, The Gathering), Memnarch, Jodah and Lim-Dûl are, in fact, not. It is uncertain if The Gathering#M is a planeswalker.

The Elder Dragons

The elder dragons Arcades Sabboth, Chromium, Nicol Bolas, Palladia-Mors and Vaevictis Asmadia once were powerful planeswalkers. They fought a large war entitled the War of the Wyrms or the Elder Dragon Wars. They lost their spark when a powerful world spell cast all other dragons to the land, creating the elder land wyrms.

Gods

Some planeswalkers are considered to be gods by the inhabitants of the planes. On the other hand it is uncertain if some of the known gods are really planeswalkers. Examples of these are:

Some characters are considered gods, but certainly aren't planeswalkers

the oft mentioned Yawgmoth
Karona
Ramos
the primeval dragons Rith, Treva, Darigaaz, Dromar and Crosis