From the introduction:
Artifacts can be created by both the immortal, and the mortal, but often, it takes immense magical energy to create an artifact, and thus they only appear sparingly in the multiverse, and only very rarely are there more than one of them in existence. Many of these are created in some of the greater planes, outside the material plane like the mithral forge, or the astral sea of Mount Celestia.
Some artifacts are sentient items as well, and often these have complex personalities designed by those who they were created by. These, like other artifacts, are rare, and often the main quest of a troupe of adventurers, or the primary weapon of a villain.