Head out to Arborstone from Altrumm Ruins. Follow the wide corridor to where the quest star on the map is pointing. You will engage several Warden groups along the path. You cannot go straight to the mark on the map, instead you must climb the stairs on either side. Beware that if you come too close to that mark from below, you will cause the assault to begin which (by the time you go around and up the stairs) will mean Terrik's certain death. His death however has no consequence as you will still complete the quest successfully if you kill the Warden groups.