Artifacts are Magic Items that can only be crafted at Construction Level 8. For reasons particular to each of them, only one of each Artifact can exist in the game at a given time; if you try to craft an Artifact that already exists, you will fail. If you Wish for an Artifact that already exists, you will be taking it from whoever had it before you.

The default game rules specify that you can only have one Mage forging an Artifact each turn, but this condition can be disabled before the game starts. Doing so has the side effect of letting someone delegate multiple mages to forge the same Artifact at the same time, for a better chance of getting it; which isn't super relevant in most games, but it can happen. Resources that go into an Artifact that cannot be crafted are wasted.

You can think of the list of Artifacts as a "ban list", like in Yu-Gi-Oh or other trading card games; that might help with understanding why some very powerful Items share this list with things that seem utterly pathetic.