If you do not anger him in your first conversation, by saying you will leave, speak to him again, say that you are looking for adventure, he will give you "Mekrath's mirror" quest.
An imp has stolen a magical mirror from Mekrath, and now resides in the southwest section of the sewers, accompanied by a lesser earth elemental, likely summoned by the mirror.
Once dispatched, take the now broken mirror back to Mekrath and you will be rewarded 18750 Exp. And his "slave" Haerdalis, you can either accept or deny, causing him to turn hostile, yielding another 14000 Exp. When defeated.
He starts out with Stoneskin already active, then casts protection from normal weapons, once dead he drops:
Mage robe of fire resistance
Mekraths Mirror (broken)
a Tzar gem, Potion of stone form, and Potion of invulnerability
Retrieve Mekrath's Mirror from his imp. (finished)
"I have completed Mekrath's task, and the wizard tried to "reward" me with a slave of his.
He says the man is in the north storeroom of his tower...
I suppose I shall go and see who this person is, at least, once the wizard has been defeated."