Kill the channel cleansers ( 123, 31, 477 ) /waypoint 123, 31, 477 until you receive all the runes. They will drop a small chest that will contain one rune. Quest will update when each rune that is needed is looted:
To spawn Thalbinous you have to kill the 3 groups of fiendish weedlings. All 3 groups have to be wiped to spawn him. If you are too slow and the first group spawns again before you killed the 3rd he doesn't spawn. One group spawns by Thalbinous, the second on one side of the main building, then cross through the alley to the other side to kill the third group. There are 4 in each group. When all are dead run back up the ice wall and speak to him.