Sealing Spike

The Sealing Spike, otherwise called the White Stone, is a sacred lock, with the purpose of maintaining the seal on the The Imprisoned in Skyward Sword. It is located in the Sealed Grounds. It is presumably destroyed with Demise when Link wishes for the destruction of the Demon King in the present, causing the Isle of the Goddess to fall from the sky and land in The Imprisoned's pit, destroying the monster.

Usage
The Sealing Spike was constructed by Hylia, to serve as a lock on Demise/The Imprisoned. At the same time, the Temple of Hylia was built in order for its sole inhabitant, Impa, to keep watch over the Sealing Spike, making sure that the seal on the Demon King would not break, freeing the monster inside. Centuries later, in order to keep the seal intact, Hylia, as her mortal reincarnation of Zelda, puts herself into a suspended animation.

The Sealing Spike not only serves as the seal on Demise, but also as The Imprisoned's vulnerable point whenever it breaks free from its prison. The Sealing Spike, which is impaled in the monster's head, can be accessed either by destroying its toes and knocking the creature down, or by using the Groosenator to launch Link directly onto the beast's head. Once he has access to the Sealing Spike, Link hammers it back into The Imprisoned's skull by smashing it with his sword. After The Imprisoned is defeated, Link must use a Skyward Strike on the Sealing Spike to restore the seal on Demise.

Trivia

 * In the game files of Skyward Sword and in Hyrule Historia, this object is solely referred to as the White Stone, however, no mention of the name occurs in-game. Instead, it is simply called a "sealing spike".
 * Demise's forehead is marked with a noticeable scar, suggesting where the Sealing Spike impaled him.
 * The final seal formed by Link resembles the Triforce.