Pandora's Box (パンドラの箱, Pandora no Hako?) is a Noble Phantasm possessed by the Pandora Class Card used by Erika Ainsworth. It is a box granted to Pandora by the gods, but she was instructed to never open it. It contained calamity and despair, although it is said that only Elpis (『希望』, Kibō?) would remain after it was emptied, and her opening it out of curiosity released all the world's evils across the land. Although she quickly shut the lid, humans came to know famine, disease, and strife as a result of it being opened. Eplis was left in the box, and although its true nature is unknown, it is interpreted as being something like "hope", "premonition", or "precognition". It is said that humans were saved from being wiped out upon it being closed. There is another theory that blessings came out of the box rather than despair.
It is being utilized by Darius Ainsworth as a Holy Grail, a wishing device created by deities from the Age of Gods that surpasses a normal Holy Grail. While the legend of Pandora exists in the Ainsworth's world, the legend of her opening the box is absent. It is not mentioned in the books of Greek history, and even a well-traveled person like Shirou Emiya has never heard of it. It is suggested that the box was never opened in that world, allowing the Ainsworths to hide their goal from the people of that world. Julian is attempting to have Erika open it, but she has failed thus far. Gilgamesh hints that he needs two Holy Grails, Miyu Edelfelt and Illyasviel von Einzbern, to accomplish the task.
Technically a pithos, it first appears as a large black floating cube floating over the Ainsworth estate, but it is later compressed 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. Only the sword produced by Tanaka transforming her hand shows the ability to stop it. Erika calls it an "arrow of fire", the world's truth that passes judgement on counterfeiters like them.
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. If left unchecked, it will eventually cover the entire world, but both Julian and Darius express that such a thing is partly in line with their method of somehow saving the declining world. Erika has some control over the mud, sending out massive tendrils with skull faces.
Upon activation, the Box gains several magic circle-like patterns and starts to spew mud from one corner. The downpour covers Erkia, who is unaffected by it, and begins to manifest countless corrupted Heroic Spirits. Julian says that they are all those who sought the Holy Grail and failed, beasts driven only by their desires to obtain the Grail. They will continuously emerge until they find it and fill the world with despair. Only the Gilgamesh Class Card was able to pass through with an undiluted ego.
The Heroic Spirits, in both their individual strength and numbers, overwhelms all of Illya's group. They can attack in a great rush, utilize Noble Phantasms, and display some coordination in the launching of a volley of arrows. No matter how many are defeated even with a weapon as powerful as Excalibur, or how much they are pushed back, more will keep manifesting until the mud is cut off at the source.