Magister Darenis wants you to travel to the Sanctum of the Moon to the west of Tranquillien and recover 8 Crystallized Mana Essences from the Arcane Devourers and Mana Shifters that inhabit the structure.
Can you believe it? We have an actual functioning arcane sanctum up and running southeast of here: the Sanctum of the Sun. All manner of important magisters have come to take part in running it.
Of course they've all forgotten about the Sanctum of the Moon. Not much remains of it and its energies have given rise to bizarre creatures.
I'm not one to let good resources go to waste, however. Go to the Sanctum of the Moon and see if you can recover the crystallized essences from the creatures there.