In the town of Beregost in front of the The Burning Wizard in the middle of town is a npc named Garrick. He is hiring mercs to protect his patron Silke. Accept his offer and you will automatically walk to where Silke is located. Save now and you can finish this small quest with both outcomes for the two achievements
Talk to Silke and 3 "thugs" show up. You are given 2 dialog choices. One for refusing to attack the men and one for attacking the men.
(Evil outcome)
Attacking and killing the men nets you Mercenary of the Month achievement, 200-400 gp, -2 rep, a few potions of defense, and a handful of low priced gems. Garrick walks away not joining your party.
(Good outcome)
Silke (a mage so becareful) will attack you if you refuse to attack the men. Killing her nets you the Worst Bodyguard Ever achievement, 400gp, a Potion of Invulnerability, and a Quarter Staff +1. Garrick will request to join your party.
You have 3 choices on how to handle the men that showed up.
You can talk to the leader and receive a Potion of Defense and then the men leave.
Before you talk to them, you can pickpocket the men to get the loot you would have gotten if you killed them plus the potion of defense for talking to them.
Or you can also kill them for 15 xp each and loot them with no reputation loss.