Tribunal:The Barbarian and the Book

Quick Walkthrough
# Speak to Thrud in Godsreach (or Dilborn in the Residential Sewers).
# Enter the Residential Sewers in Godsreach with Thrud.
# Speak to Drathas Nerus to learn that Dilborn owes him 3000 gold.
# Use Speechcraft to lower the amount to 1000 gold.
# Pay it off from your own pocket.
# Speak to Thrud to receive your reward.

Walkthrough
A Lost Friend
In the center of the Godsreach District you'll find the simple-tongued Thrud wandering around looking for his lost friend Dilborn. Inquire further and you'll learn that Dilborn the wizard has been missing for three days now and that he was last seen entering the sewers in the area. Thrud will insist on helping you find his friend and will not stop following you once you begin this quest. 

Rescue Attempt
The sewer's entrance are in the northwest corner of the Godsreach District. You'll find Dilborn and three of his "friends" not far from the sewer entrance in the central area of the Residential Sewers, at an eastern dead-end. Dilborn has nothing interesting to say except for a few cries of help, and the nearby goons will direct you to their leader, Drathas Nerus. Apparently the wizard owed Drathas money, and he is determined to get it one way or the other.

If you attack Drathas, his henchmen will attack and kill poor Dilborn. If you are quick and skilled enough, however, you can attack and take the three of them out with help from Thrud. Attacking either of the henchmen will cause them to focus their attacks on you, rather than Dilborn. Drathas has a weapon that paralyzes for twenty seconds, so try to keep your distance or have anti-paralyzation enchantments or potions.

If you choose to pay the money Dilborn owes, speak to Drathas to find out the sum is a whopping 3000 gold. If you ask to lower the fee, and as long as your disposition with him is high enough, you can get it down to 1000 gold. If you threaten him he will order his men to attack Dilborn (who doesn't last too long). To regain your money it is possible to kill Drathas and his gang after rescuing Thrud and Dilborn; Drathas has a few nice items.

You are rewarded by Thrud with the skill book Trap. 

Notes
It is possible to complete the quest without speaking to Thrud. You can free Dilborn on your own, but you will not receive the reward if you choose to do it this way. Thrud de-spawns, if he's not in the sewers with you.
You can use a form of Command Humanoid to maneuver Dilborn out of harm's way, before dealing with the thugs.
You can cast frenzy one by one on the captors and the cohorts will stand idly by.
If your character isn't the greatest and you have problems taking on all three of the gang, do the quest as normal by paying Drathas but then walk away and sleep for a while (the length doesn't matter) and return. Drathas and Dilborn are still there but the cronies have left and you can then kill Drathas for some rather nice glass boots and greaves, plus your money back. 
If you still have any PoisonGrip Arrows, they can make for effective crowd control.
It is also possible to complete the quest, get the skill book, then kill Thrud and take his Ebony War Axe worth 15k as repayment for Dilborn's freedom.
With a high enough Sneak skill it is possible to complete the quest by killing the bandits one by one. If you attack one of them while sneaking, the remaining bandits won't react, and the gang leader will have the dialogue option to pay him the money even after you kill his henchmen.
 

Quest Stages