Unlimited Blade Works

Infinite Creation of Swords.
Though it is treated as a Noble Phantasm, technically speaking it is a type of magecraft called a reality marble.
Within this reality marble exists all the ingredients needed for the creation of swords. So long as Archer has seen the original, he is able to easily replicate it.
However, the replicated weapon has its rank reduced by one.
Replication of defensive armaments is also possible, but it requires two to three times more magical energy than a normal projection.
Once an armament has been replicated, it is registered inside the boundary field and can be manufactured with Gradation Air even without expanding the reality marble.
Against a normal opponent this is little more than a mildly annoying ability, but it is the natural enemy of a certain King of Heroes.

Fate/side material: Fate Encyclopedia

How many times have I’ve gone over this, Nausicaa?
This is Archer’s Noble Phantasm. It’s a barren wasteland with a thousand swords scattered about.
A great ritual deployed using eight lines of projection.
It’s a kind of old world magecraft known as a Reality Marble. A Reality Marble materializes the user’s inner world and overwrites the real world with it for a short period of time.
All the weapons that Archer can project are present in this space, and there is no need for him to project anything from scratch. He can immediately use any weapon that’s sticking in the ground.
In EXTRA, since the requirements for producing wrought iron have increased, Holy Sword class projections are possible (although their power is inferior to the originals).
…Well, what really happened is I took my eyes off the 3D art team for a second, and when I looked again I was surprised to find they had made an animation of “Excalibur Projection.” Since they put the effort in to make it, I figured might as well include it. As long as the Holy Swords are inferior to the originals I deemed them to be borderline safe as far as the lore goes.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA