The Noble Phantasm hidden within the Sealing Mystic Code Add that Gray possesses. Though it is called a spear, its true nature is as the “shadow” of an anchor that fixes the Texture of the World in place.
In order to activate this Noble Phantasm that was once even wielded by King Arthur, a tremendous amount of magical energy is required. In addition to that, the Thirteen Strictures placed upon it are still in good condition, meaning it’s true power cannot be exercised without a majority vote from the Round Table.
In the Rail Zeppelin incident, five such votes were obtained. The names of those knights are ones Gray always remembers with thanks.

“This is a fight for survival.” — Acknowledged, Kay.
“This is a fight against someone much greater than I.” — Acknowledged, Bedivere
“This is a fight where I will not turn my back on my humanity.” — Acknowledged, Gaheris.
“This is a fight for Truth.” — Acknowledged, Sir Agravain.
“This is not a fight against Elementals” — Acknowledged, Lancelot.

Translator’s Notes
  1. ^ The Lance that Shines at the End of the World, also translated as “The Lance that Shines to the End of the World” and “The Spear that Shines at the Ends of the World.”
Lord El-Melloi II Case Files material: Lord El-Melloi II Case Files Glossary