Pandora's Box

is a Noble Phantasm possessed by Pandora. It is being utilized by Darius Ainsworth as a Holy Grail. Technically a, it first appears as a large black floating cube floating over the Ainsworth estate, but it is later reduced to a small cube able to fit in Erika Ainsworth's hands. It is extremely durable, able to withstand Kuro's Noble Phantasm arrows without a scratch. The first thoughts of someone looking on is that it is "not meant for human hands", and it is related to the reason Tanaka wishes to destroy the Ainsworths.

Mud
Pandora's Box can endlessly spew out mud, Julian calling it the "Ainsworth's darkness" that will spread out enough to turn the world into a prison. The mud is capable of manifesting countless corrupted Heroic Spirits, all those who sought the Holy Grail and failed. They are beasts driven only by their desires to obtain the Holy Grail, and they will continuously emerge until they find it and fill the world with despair. They have both individual strength and numbers, overwhelming all of Illya's group without them being able to even damage them. Unable to even buy time against the horde, only Shirou displays any ability to damage them. They can attack in a great rush, and display some coordination in the launching of a volley of arrows.