Find Arbos. The quest update is located at ( 669, -10, -380 ) /waypoint 669, -10, -380 in the Elddar Grove. This is the tree with the lift/elevator.
Head to Enchanted Lands to investigate Arddin (a tree in Misty Thicket). It's located at ( -592, 8, -737 ) /waypoint -592, 8, -737 (near Foomby). You will be attacked by three or four Corrupted spirits (level 38, 2 down arrowed mobs - if you stay near the tree these mobs will keep spawning).
After this you need to kill appr. 15 evil nymphs in The Enchanted Lands, which are the nymphs in the area surrounding the tree.