Talk to Dante Sion (2848,1206,0) in the rat pen south of Lighthaven: say “rats”, “surprised”, then “yes”.

He needs 10 goblin hearts, which can be found on the goblins around the city, to feed his rats.
Bring him the 10 goblin hearts and say “reward” then “offer”. The next times, just click on him.

You can complete this quest as many times as you want.

Rewards:

• 3,200 XP
  * 1 Gem containing a rat
  * 1 Potion of severe wound healing
  * May also give a Desert Bone Bow (but not every time)