Mirror, Mirror  

'It took a bit of doing, but I have proven my theory about the crystals. The blue ones do, in fact, react to light reflected from the mirrors!

'There are some unbroken crystals mounted near a mirror I examined a few days ago. To find it, descend from here, take the west passage, and descend further. Follow the path until you come across a four-way crossing of mining-cart rails, then follow the left path east and down until you reach the stone cavern-floor.

'I would like you to go and swivel the mirror so that its light is directed at them and see what happens.'

Thettmarr believes that, with the adjustments to the positions of other mirrors he made, a mirror he previously examined might light up nearby crystals.

Objective 1
Swivel the mirror
To fond the mirror start from the Deep Descent, head down, and take the western passage. Follow the path until you come to a four-way crossing of mining-cart rails, then follow the left one east and down until you reach the stone cavern-floor.

Thettmarr has asked you to swivel the mirror in the caverns below so that the light strikes the crystals he described.

Objective 2
Talk to Thettmarr at the Deep Descent encampment
Thettmarr is in the Deep Descent camp.

You should tell Thettmarr what happened when you swivelled the mirror.

WARNING
When you swivel the mirror, you will summon and be attacked by a Grodbog Hive-queen, you do not have to kill it, to complete this quest, you can defeat it, or run away.