I just tried the Harbinger mission to get all the feathers to complete my lorebook. However when I went to get the fourth feather it simply did not exist. I consulted numerous guides and found that I was not the only one experiencing this as many others have not been able to complete this Triumph in recent months. I myself was on the second week rotation and could not locate feather #4 despite it supposed to be where the hobgoblin spawns upon entering the Taken portal.