Reward: Treasure Clue
Guild Master will contact you through the Guild Seal and tell you there are some Twinblade fanatics wanting you six feet under and you can expect some attacks. Nothing surprising, really. It happens mostly sometime after killing the White Balverine in Knothole Glade. Ironic, that you have to search for the assassins yourself, and not the other way round.
Assassins appear when you are at the one certain spot of the following maps. You will have to wander around them for a while.
1) Knothole Glade (next to the bridge);
2) Windmill Hill (at the floor of the windmill);
3) Witchwood Cullis Gate (next to the stone cullis gate in western part of the area);
4) Prison Path (on the main path);
5) Hook Coast (next to the city bell).
When you kill all five of them, you'll get yet another Treasure Clue.