Artifacts are unique items that give you a permanent bonus when assembled. You can find them in the Artifacts menu at the bottom of the main screen after you reach level 8. Each of the artifacts consist of 5 different items that you can get as rewards for patrol missions. Artifacts are created in the Workshop menu and have 4 different quality levels: common, rare, epic, and legendary. The higher the level, the better the initial bonus is.
When choosing one of the pieces from the main Artifacts screen, you can see the description of its bonus, change the active artifact of that kind, or upgrade it. Upgrades increase the bonuses and are done with the help of other artifacts, blue stars, and special card suit items. There are four kinds of card suits that you can get from fighting bosses.
Each artifact type has certain upgrade limit. Common artifacts can be upgraded up to level 10, rare - up to level 20, epic - level 30, legendary - level 40.