The Others, known as the White Walkers to the Free Folk, and rarely the Cold Gods and the White Shadows, are humanoids originating in the far north of the continent of Westeros, beyond even the Land of Always Winter beyond the Wall. They are considered mythical by most of the people of Westeros. Despite what is thought, they are not dead.
According to legend, the Others first appeared approximately eight thousand years ago out of the extreme north. At this time peace reigned in Westeros due to the Pact between the Children of the Forest and the First Men, and they were unprepared for the scale of the invasion. The Others' arrival was preceded by a winter that lasted a generation and a period of darkness known as the Long Night, although it is unclear if this was a literal period of darkness lasting years or merely an artistic term.
The Others overran much of Westeros and proved to be almost unbeatable in combat, able to resurrect fallen enemies and even dead animals as wights to serve them as utterly loyal foot soldiers. The First Men and their allies were pushed back over a long and gruelling war.
Salvation only came with the discovery that obsidian, also called 'dragonglass', could slay the Others, and with the arrival of a mighty hero to lead the war. This hero is known in the east as Azor Ahai and was said to wield a sword of fire called Lightbringer. In the War for the Dawn, Azhai led the crusade that turned back the Others and saw them banished from Westeros and back into the uttermost north. After the war was concluded, Brandon Stark built a huge wall of ice to prevent the Others coming south again and founded the Sworn Brotherhood of the Night's Watch to keep guard against this eventuality. Brandon also established the castle of Winterfell and became the first King in the North.
The children of the forest placed spells within the Wall which were said to prevent the Others from passing under or over it by any means. After the War for the Dawn was concluded, the already small numbers of the children slowly declined until they had all disappeared around the time of the Andal invasion.
Over time, the threat of the Others receded, and they came to be regarded as either just legends, or a real race long since rendered extinct. However, the Night's Watch remained in place, although it now considered its purpose to be preventing the wildlings from entering the Seven Kingdoms and causing mayhem there.
Starting several years before the War of the Five Kings, the northernmost wildling tribes and the rarely-seen giants of the furthermost north began to suffer attacks from an unknown enemy. Unable to stand against these forces, they began moving south in ever-greater numbers. The King-Beyond-the-Wall, Mance Rayder, began gathering these forces into one host. Once it became clear the unknown attackers were the Others returning after millennia, Mance realised that the wildlings could only survive by taking refuge south of the Wall. Aware that the Night's Watch would not listen to reason, Mance planned a massive assault on the Wall. He had also taken possession of a device known as the Horn of Winter, which according to legend could shatter the Wall. He was unwilling to do this since it would also allow the Others to follow them into the Seven Kingdoms, but if there was no other way of breaching the Wall he was prepared to employ it.
Thanks to the efforts of a young steward named Jon Snow, Mance's plan was thwarted, and his forces were delayed at the Wall long enough for King Stannis Baratheon's forces to arrive and rout his host. Stannis agreed to allow the surviving wildlings to cross the Wall, but only if they swore fealty to him and agreed to reinforce the Wall and help defend it against the Others. Backroom dealings were made and eventually, they reluctantly agreed.
Motives and beliefs
The Others speak a language alien to the people of Westeros called Skroth, and no communication has been possible with them. The Others' motives thus remain completely unknown at this time.
According to the red priestess Melisandre of Asshai, the Others are the servants of a deity called the Great Other, the god of darkness, ice, and death who is locked in eternal warfare with R'hllor, the god of light, fire, and life.
A Game of Thrones
In the Prologue of A Game of Thrones, three brothers of the Night's Watch, Will, Gared and their commander Ser Waymar Royce are tracking wildlings north of the Wall. Gared stays with the horses as Will and Ser Waymar investigate a group of wildling corpses, but find that they have mysteriously disappeared. While Will is scouting up a tree, a small group of Others appear. Their armor is eerily beautiful and changes hue depending on how the light catches it, and they carry blades of inhumanly sharp crystal. Ser Waymar bravely challenges one of them, but his sword shatters and blinds him. The Others then slash at him, killing him mercilessly. After a long time, Will finds the courage to look up and sees that the Others are gone. Then Ser Waymar's corpse stands up and begins to strangle him, his eyes glowing bright blue, his touch icy cold.
A Clash of Kings
Gilly, one of Craster's daughters and wives, is brought before Jon Snow by Samwell Tarley. Sam wishes to help Gilly as she is pregnant and is concerned that if she gives birth to a boy, Craster will sacrifise it to the Others. It is later confirmed that Craster does indeed take his sons to the Others, whose purpose with the babies is unknown.
A Storm of Swords
While waiting for search parties to return, the Great Ranging of the Night's Watch are attacked on the Fist of the First Men by hundreds of Wights. Eventually, the surviving black brothers flee for the wall, pursued by the Wights. Some of these survivors claim to have seen the Others during the battle.
Samwell Tarly, Grenn, and Small Paul have fallen behind the bulk of the survivors, as Samwell has collapsed from fear and exhaustion in the snow. The falling snow thickens, and the air suddenly grows much colder. Out of the gloom appears an Other astride a dead horse. It dismounts and attacks and kills Small Paul first. As Small Paul dies, he falls on the Other, disarming it. Samwell takes the obsidian dagger that Jon made for him and stabs desperately at the Other. The Obsidian causes the Other to melt away, armour and all. This earns Sam the name Sam the Slayer.
Later, Stannis Baratheon arrives at the Wall with Melisandre of Asshai, an army, and many of his bannermen. Stannis claims he has come to do his duty for the realm, to defeat "the only enemy that matters". He means to combat the Others and their god, the Great Other who Melisandre claims is the eternal enemy of R'hllor and the cause of all evil and darkness - the Others are merely his thralls.
Appearance and abilities
The Others are tall, slim humanoids with cold blue eyes and chalk-white skin. It is unknown whether they bring the cold and darkness with them, or if they simply choose to show themselves only when it's cold and dark. They wear reflective armour and wield thin, crystal swords. These swords can freeze and shatter any object they touch, including the steel blades favoured by the Night's Watch. The Others appear to be superior swordsmen.
The Others have the ability to return creatures they slay to life as wights. These wights do not appear to need food or rest and can operate with purpose and some rudimentary intelligence. Whilst the Others themselves do not appear able to cross the Wall, their wights show no such limitation.
The Others have so far only appeared at night and in times of great cold, which is consistent with their previous invasion apparently only being possible during a ferocious winter and during a time of great darkness. The Others are believed to thus be vulnerable to fire, particularly extremely hot fire such as that breathed by dragons, and possibly to daylight as well, although the latter is only conjecture.
A book from the library of Castle Black also states that Dragonsteel is another way to kill the Others, Dragonsteel being Valyrian Steel.
The Others also have a proven weakeness to obsidian, and when slain appear to melt into ice water.