This is story-related and will be unlocked automatically after rescuing the Archaeologist from Minions.
TIPS
* There are two types of minions: Magical and Melee, so be prepared for distance combat and up close.
* The final section of this quest is TIMED, so you will need to dispatch several groups of Minions quickly. It is also worth noting that some Minions will retreat back up the path you work your way down, meaning if you don't dispatch them quickly, they may cause you to backtrack, costing time. There is a checkpoint right before the timed section, so failure isn't especially punitive.
* You will have some assistance from Guards, but they tend to get in the way (especially when using any Area of Effect spells). If you're concerned about your Morality, you may want to take notice.