Unlimited Blade Works

It is treated as a Noble Phantasm for the Heroic Spirit Emiya, but to be more accurate, it is a Reality Marble, a type of magecraft; it is also the Reality Marble of Emiya Shirou. Although, the worlds of Emiya Shirou and Emiya are different.

All elements to create swords are present, and an original that has been seen can be easily reproduced. Arms reproduced once are registered inside the bounded field, and they can be produced through Gradation Air magecraft even without activating the Reality Marble. However, reproduced weapons lose a rank.

It is also possible to reproduce defensive armament, but the prana cost is two to three times higher than a regular Projection. Furthermore, armament prepared during the formation of the bounded field keep consuming prana from the formation to the preservation of the bounded field. The prana cost is also brutal when recreating those that were destroyed or when creating those that didn't exist when the bounded field was created.

It is more or less a burden for a Servant who "carries" a weapon, but against Gilgamesh who "owns" the weapons, it has the advantage of having weapons ready in proportion, making it his natural enemy.

Source: based on the Fate/Side Material encyclopedia entry (p.76) and the dictionary resource at Jonnobi.