There are five assassins to kill. Here they are in no particular order.
The Prison Path; You will find this disciple of Onan about halfway down the path. One down, four to go.
The top of the windmill on Windmill Hill; Just climb the stairs to the roof of the windmill. He will then charge up the stairs to engage you.
Two down, three to go.
The next two can be found on Witchwood Island. The third assassin will attack you when you wait behind the Witchwood Focus Site. Wait a minute or two, then he will attack.
Three down, two to go.
The fourth assassin will meet you after you cross the bridge near the Hidden Copse demon door, just before Knothole Glade itself.
Four down, one to go.
The final fool will try to sucker punch you when you go round the back of the Hook Coast town bell.
ALL DEAD.
Killing each assassin yields a reward, such as gold, doll of you, etc. Kill them all, and you will receive a gold reward from the Guild, and a treasure clue.
I hope this helps. Happy hunting!