If you are like me and you just couldn't be fkd to spend hours inside a scenario for the hidden appearance for unholy then you probably missed it when you finally unlocked it.
Run this macro. /script print(IsQuestFlaggedCompleted(44188))
If it says true, you already unlocked it. Just head to icc putricides room on the right is a teleportal.. go in and kill the final abom and boom you got it.