Artifacts are rare and powerful items found within OASIS. Each artifact is unique and has its own special power. They are usually obtained by killing the final boss, and sometimes appear as random drops from high-level bosses.
Only one of each is obtainable as far as characters in the novel know, with the exception of the Easter Egg keys (i.e. each avatar who accomplishes the respective quest gets a copy).
Artifacts are described to be indestructible by conventional means (i.e. the Cataclyst is a one-use artifact which is destroyed after its detonation). When the Cataclyst was detonated in Sector 10, Parzival (after being resurrected by the Extra Life Quarter) was able to find and use artifacts lying around the crater.
Although there are presumably hundreds of artifacts throughout the OASIS, only a handful have been explicitly mentioned in the book