Sealing Spike

The Sealing Spike is a sacred lock with the purpose of maintaining the seal on the the Imprisoned in SS undefined. 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.

Location and Uses
The Sealing Spike was constructed by Hylia in the distant past to serve as a lock on The Imprisoned. In the present era, Zelda, the mortal incarnation of the goddess Hylia, travels through the Gate of Time and arrives at the Temple of Hylia, at a time where Demise was only recently sealed. Following the plan originally devised by Hylia, Zelda puts herself into a suspended animation in order to sustain the seal placed upon Demise, while Impa becomes the guardian of the temple, watching over Zelda during her slumber.

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, this object is solely referred to as the White Stone; however, no mention of this name occurs in-game. In addition, there is a file named "Black Stone" consisting of an unused model similar to the Sealing Spike. Given its name and its appearance, it may have been intended to be a counterpart of the Sealing Spike.
 * Curiously, the Sealing Spike is not present in the past during Ghirahim's ritual, and its absence is never brought up or explained.
 * Demise's forehead is marked with a noticeable scar, suggesting where the Sealing Spike impaled him.
 * The symbols on the three sides of the Sealing Spike are the Marks of the Goddesses.