Void

The Void is an inky-black substance that is known to exist in the Abyss. It plays a major role throughout the history of the Kingdom of Hallownest.

The Void was used by the Pale King in his experiments to create constructs. His laboratory within the White Palace houses moulds for these creations as well as their shells seen on Kingsmoulds and Wingmoulds. A peculiar mould found there suggests that the The Collector was also created here.

The King experimented and created numerous Vessels to seal away the Old Light, the Radiance, however most of these were dubbed failures, and then dumped into the Abyss.

It is possible that the Soul Vessel plays a part in sealing the Void within the husks the King granted the Vessels, based on the fact that when the player dies, their Soul Vessel breaks and their Shade is let loose. The Shade, as well as Siblings found within the Abyss, are the "fragment of a lingering will".

Large towers were constructed in the Abyss, reminiscent of lighthouses, that used light to repel the Void.

After many rounds of trial and error, finally, a Vessel was selected to seal away the Radiance and the entrance to the Abyss was sealed with the mark of the King, stating that "we shall enter that place no longer."

Constructs
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 * Void Tendrils - Possibly the King's first experiments with Void. The desired shape was first outlined with a mould and then filled with Void.
 * Kingsmoulds - Constructed along with the Wingmoulds in the King's workshop.
 * Wingmoulds - Constructed along with the Kingsmoulds in the King's workshop. Unknown how the shape was created, although several of their armour is found hanging from the ceiling.
 * Siblings - The Vessels that would seal away the Radiance, including the Hollow Knight. Thousands upon thousands of these were created, only one proving to be the pure enough.
 * The Hollow Knight - The first "pure" Vessel.
 * The Knight - The Vessel the player controls throughout the game
 * Hornet - Possibly. Hornet herself mentions that she is similar to the player and the Hollow Knight, made by the Pale King as a gift to Herrah the Beast; however, the details of this process are never clarified and she states that she is different from the Vessels.