As you venture forth through the land and end up in Windhowl, you will talk to NPC's in the town and will realize rather quickly that something is amist.
There is a wounded guard there who is in great pain and claims that that the troll has hurt him severely. He is in the first house on the left as you enter into Windhowl. He asks you to kill the troll.
This is a simple quest but finding the ugly sucker and defeating him is another matter altogether. The road is long and the way is surrounded by some of the toughest monsters on this island.
Anyway, kill the troll and return to the wounded guard and say troll. He thanks you for ridding the land of the vial monster and in return gives you a Shield for you troubles.