Speak to Nikora. She is found behind a rock at the top of the ramp to the Blackshield dock ( -380, -51, -860 ) Copy/waypoint -380, -51, -860.
Slay 20 blackshield assassins (level 12) southeast of Nikora. ( -376, -51, -863 ) Copy/waypoint -376, -51, -863 There is also a group of them near ( -489, -48, -756 ) Copy/waypoint -489, -48, -756.