You can initiate the quest, once your party has access to Kalrathia in Act III, Chapter 2 (Restore Kalrathia's Water). Head downstairs from the Kalrathia teleporter, and turn southwest and enter the room above where Enchantress Valeria, Mage Boden, and Archanist Bianca are situated. Turn south and walk down the stairs and speak to Mage Boden to initiate this quest and he'll give your party a golden mirror and some clues as to where his apprentice might be.

You can complete the quest, once your party has access to Darthrul in Act III, Chapter 2 (The Morden Chief). Either go there as part of the primary quest or teleport there afterwards. There's a pathway just outside the front gate, which your party can take by heading west around the western gatehouse and turning north, then descending the stairway into the dry canal. Walk a short distance to the west, then climb the second set of stairs and head southwest toward the entrance to Some Ancient Ruins. Walk down the stairs and enter it.

An examination of the seventeen statues holding mirrors in the large room, reveals that the statue farthest to the north with a beam of light falling upon it, is missing its mirror. Left click on that statue to install the Golden Mirror, given to your party by Mage Boden. Doing so causes the downward light beam to be reflected into the room. The object here is to cause the reflected light beam to first shine on the door to the north to open it and then rearrange the statues so that the light beam shines on the door to the west, opening it. It's not as difficult to do as it looks.

The north door is easily opened by directing the light beam first south one statue (1 - 2), then west one statue (2 - 3), again south one statue (3 - 4), and east one statue (4 - 5) and finally to the north, forming a square, to unlock the door beyond. See below:

Once accomplished, head on into the north room and speak to Mage Boden's apprentice, Darek, to learn the truth about the nefarious dealings of Mage Boden and to receive Darek's Journal and obtain the Greater Chant of Fighter Power from the lectern on the east side of the room and the Greater Chant of Casting from the lectern on the west side of the room. Return to the large room and rearrange the statues and mirrors like this to open the west door. Enter the room to the west and open the big chest, plus other chests and barrels for some decent treasure.

Teleport back to Kalrathia and head downstairs, then turn southwest, approaching the doorway, and enter the room above where Mage Boden is situated. Turn south and walk down the stairs and speak to Mage Boden. He'll demand that your character return his journal. When your character confronts him with his duplicity, he attacks and the only thing to be done is to destroy him.

It should be an easy battle in Mercenary mode. He has nearly 100,000 Hit Points at level 59 in Veteran mode and it takes a while to take him down. Once it's over, Mage Apprentice Darek then returns to Kalrathia and sets up shop outside his former Master's place of business, which is just down below the teleporter to the south, or he may immediately return to the shop and take his place where his former master did business.