You can access this area depot +3.

The Faith Servant will give you a quick run down of what is happening…

Faith Servant: Greetings young servant, I am an intermediary between this world and the immortal world. War is waging between Heaven & Hell, and I can help you defend your Faith, first you must complete an arena for the Faction you are prepared to wage war against. If you have completed an Arena, you may access that Factions home base and purchase artifact. Once you arrive at your factions home, you should first speak to their porter and travel to the frontlines. Simply say depths of hell or cathedral of heaven.

*All underlined words are key words.

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.

Keep in mind this costs AT and Gold for every attempt, and if you die you must restart from the beginning.

Depths of Hell (For those with the Hell Artifact) or Cathedral of Heaven(For those with the Heaven Artifact) will take you to the main base. This is where you get your dailies that you can complete at the frontline or the horde caves or the sanctuary. Talk to the Demon Gateporter or Angel Gateporter. Completing these tasks will level up your Artifact.