Heaven's Dress

A magical artifact passed down by the Einzberns.
As the heart that governs the Great Holy Grail, it is imbued with magecraft that can materialize the soul for just a few seconds.
Though the dress is pure white, it is actually made of gold. It is inlaid with seven “rings” said to control creation, and the souls of others.
The seven holes in the dress are the seven transformed rings, and they are designed to light up one by one every time a Servant's soul is harvested.
Since any human who touches the dress will be turned to gold, its management must be handled by elementals, demi-humans, or homunculi.

Fate/side material: Fate Encyclopedia