This is the final quest for each of the four classes. It is given by your respective class trainer.
You need to head to Grimgash's Lair and kill Grimgash the Black, the leader of the Gruttooths. The lair can be found in a cave to the northeast of town.
Grimgash is only level 5-6 but he is accompanied by 5 other mobs in the lair. You will get them three at a time. When you fight Grimgash kill the Witch Doctor first or he will heal Grimgash during the fight.
Kill him and return to Braksan to complete the quest.