Reward: Skorm's Bow, Necromancer title, youth.
You will find the path to The Chapel of Skorm in Darkwood Camp. It is a place where you can sacrifice people in order to get lots of negative alignment points and some valuable gifts. The most important of them is Skorm's Bow, definitely the best ranged weapon in the game. Getting it requires some plain luck, though.
Hire two bodyguards (you will find one of them in each bigger city, it's easy to do fast because once you hire one of them, he teleports with you when you use the Guild Seal) and go to the chapel. Wait until it's about midnight (look at the indicator in top-right corner of the screen) and talk to one of the monks. Sacrifice both assassins.
HINT: Chances of getting the best prizes this way vary depending on you character's alignment (it's easier when you're good). There's no one sure way to get them, so you'll have to try it a few times (with a bit of luck you'll get the bow the first time).
Once you get the bow, sacrificing the next person will give you youth (a little change in your Age stat, better save this opportunity until late in the game when you're 65), and the second one will result in getting the Necromancer title. Neat.