The Wyrms were a species of creature that seemed to exist before the birth of Hallownest. At a certain point, the entire race died out, as implied by Bardoon: "with its like gone, the world is smaller." Only one corpse is found, and this is the shell in Kingdom's Edge.
It is unknown if all Wyrms looked the same, but the corpse that is found in Kingdom's edge is extremely long, the player being able to travel quite some distance inside the corpse before being cut off, and seems to be limbless, as implied by Bardoon. The corpse looks like it is very sturdy, and the "mouth" seems to be ringed by eight pointed growths.
The Wyrm in Kingdom's Edge is the birthplace of the new monarch of Hallownest, the Pale King. Bardoon states that "what is death for that ancient being? More transformation methinks." This implies that after the Wyrm died, the Wyrm took on a different, but smaller, form, born with a strong intellect and brilliance that the other bugs did not have. This may imply that Wyrms were intelligent beings.
Image to the right: The presumed place where the Pale King entered the world from being reincarnated, at the end of the body (or as far as the player is allowed) of the Wyrm's corpse. This is supported by the definition of the King's Brand (acquired from this place at the end of the shell), "Hallownest shall recognize a new ruler," the previous ruler being the Radiance.