Talk to the old magician's spirit. Kill Deadeye. Return to the old magician's spirit
If you cast Spirit Vision at the Abandoned Sawmill, you will encounter the spirit of a deceased mage. He asks for your help in getting revenge against the Lone Wolf archer who killed him, Deadeye. After you manage to kill Deadeye (and likely other Lone Wolves, since it will probably antagonize them as well), return to the spirit for your reward.
There are different rewards:
Tell him you like to ...air...your grievance. (Pressure Spike)
Like to splash the pain around.(Global Cooling)
Tell him you seek earth-shaking power (Earthquake)
Claim your fee in Source, his. (Gives you a temporary +1 Source Point)