He is an ancient Wyrm who burrowed through mountains and across the wasteland until finally stopping in Kingdom's Edge. Once there, the Wyrm shed its shell and transformed into the Pale King. The pale, broken egg found within the Cast-Off Shell hints at this rebirth. Consequently, the Pale King is often still referred to as "Wyrm", as this was his former body. He reduced his form in order to match the bugs of Hallownest.
The Kingdom of Hallownest
After his transformation, the Pale King expanded the minds of several species of bugs in Hallownest, granting them sapience. The Pale King wanted these bugs to devote themselves to him, and in return, he promised them that the Kingdom of Hallownest would last eternally.
The Pale King was often depicted as an imposing, gleaming figure with a fiercely horned crown. He was bright and radiant in visage, with the Delicate Flower's pure aura being no match for his light.
Although he was an elusive, reclusive figure who was rarely seen outside his Palace, the Pale King was deified by his citizens. The bugs of Hallownest believed that he created their world and everything in it. Worship was offered through King's Idols. Even the moth tribe turned their backs on their creator, the Radiance, and started worshipping the Pale King instead. The Pale King's ability of foresight, the ability to see the future to some extent, caused the mushrooms of Fungal Wastes to accept his rule as well.
When the Radiance tried to be remembered again after being almost entirely forgotten, she ended up causing the Infection. To combat this, the Pale King came up with the idea to use a Vessel to seal her away. These Vessels are the children of him and the White Lady, born in the Abyss with Void inside of them.
Three bugs were asked to become Dreamers to form seals on the Temple of the Black Egg which would contain this Vessel. One of them, Herrah, only accepted his proposal if the Pale King had a dalliance (sexual relationship) with her which would result in a child; this child ended up being Hornet.
Nevertheless, the chosen Vessel, the Hollow Knight, was unable to contain the infection for eternity due to their purity being misjudged. The Pale King raised the Hollow Knight and formed a bond with them, which may have resulted in their "tarnishing".
At a certain point, after the Infection reappeared, the Pale King and White Palace disappeared.
White Palace can be accessed in the dream realm after the Awoken Dream Nail is used on the corpse of a Kingsmould in the Palace Grounds. The Pale King’s corpse can be found on his throne in the throne room. When his corpse is struck, it falls off the throne and his half of the Kingsoul drops on the ground.
↑Bardoon: "This ashen place is grave of Wyrm. Once told, it came to die. But what is death for that ancient being? More transformation methinks. This failed kingdom is product of the being spawned from that event."
↑Mask Maker: "Has it witnessed that truth most tragic? The Wyrm's great shame sealed away."
↑Pure Vessel Hunter's Journal entry: "Chosen vessel, raised and trained to prime form."
↑Official manual:"As Hallownest fell to ruin, its reclusive king abandoned his subjects and went into hiding."
↑Relic Seeker Lemm: "Have you visited the remnants of his palace? It's down below this city, in the bedrock of the kingdom. Must've been an impressive sight in its time. Now there's nothing left. It's a strange thing though. There are no signs of conflict around the area. It's as though the whole place just vanished."
↑Wanderer's Journal, p. 124: "No actual palace structure stands there [...]. It would seem he somehow took it with him when he fled."