Tales of Arise

This is unlocked by defeating the boss in the sub quest "Spirit Temple." The optional temple has lvl 60 enemies and the boss itself is lvl 65 (higher than the final boss of the story) so you will want to leave this for post game.

Receive the quest from the "Anxious Man" in Traslida Highway, he is standing beside a wagon just west of the campsite in the middle of the map. Head north from there and enter Tietal Plain where you will find the portal to the temple.

You will need to battle enemies as you make your way through more portals. It might seem like you are lost and you can't check the map since you are continually in battle mode, but if you keep heading forward you will make it to the boss room eventually. NOTE: you can leave this boss arena and are able to re-enter without going through the temple again.

You will fight similar enemies from the temple but will have to dodge lasers from the Grand Gnome that are telegraphed on the floor. After you take out the mobs, a dragon will spawn and more mobs will be summoned when his health drops. Take them out and then stick to the side of the arena and take out the dragon until the Gnome is exposed and you can rush it down. Leave the temple and report back to the man for an artifact and cash.