The Seven Farmers Quest can be found to the South East of Krokoditopolis, I would recommend you only attempt this Quest if you are a Level 26+.
When you start the quest you will be escorted up a hill, on top of the hill you will also find the Horus Stone Circle, which will assist you in unlocking:
Once you have done the Stone Circle, talk to the girl and she will instruct you to return to her brother.
Once you return back to the town you will be instructed to prepare for the rebels, this part is relatively easy, dry grass will be nearby where the rebels spawn, light it on fire and most of the rebels should die.
The next step is to gather information from the Rebel Camp, if you are lower then Level 30 I would suggest you go stealth on this part. I got the information and only had to kill 2 rebels.
The last part of the side-quest is to find the Ancient Tomb, when you enter the tomb you will see a pit of water, dive into the water and swim to the staircase. Up ahead will be a series of platforms that you have to jump on to lift the next, you will have to do this part quick so you are able to make the jump. In the next room you will see the rebel leader and his guards, kill the guards and assassinate the leader and the achievement is yours.