This quest requires knowledge of the ogre language.
Muzmog in Big Bend (at the outdoor pub in city center) will reward you with 5 of Muzmog's Fetid Ales if you can manage to kill ten rats for him.
This can be Puny Squalid Vermin in the The Ruins or Ruin Rats in The Sunken City (it varies). It's basically a kill task is repeatable.
Completing this quest opens up the Funny Rat Smashing quest.