Can't Complete Into the Nightmare
If you have completed one of the Into the Nightmare quests but you can't find the NPC to turn it in, make sure that NPC is not assigned to a follower mission, or assigned as your bodyguard. If they are, wait for the mission to finish or unassign them, then leave and re-enter your class order hall to turn in the quest.
If you've accidentally deleted the quest item, use the Item Restoration service to restore it. If you're not able to restore the item using the Item Restoration service, you will need to abandon the mission and wait for it to come back.Note: There is a current issue causing the quest marker to occasionally show up as grey, ignore it and turn in the quest normally.