Talk to the Faith Servant, say Hi, Arena, and pick between one of the two choices. Heaven or Hell. The arena is 15 rounds, with the 15th round being a boss. Once you complete it, you will be rewarded with your Artifact.
Note: Keep in mind this costs archlight tokens and gold for every attempt, and if you die you must restart from the beginning.
There is no time limit, you can kite to stay alive longer.