I have used a Nothic in one of my games for some pretty good effect. My players found a mysterious library filled with strange books which had a Nothic inside. They say something in the shadows with their perception checks and then the Nothic used it's strange insight on them. The strange check is what got them hooked especially when I made them tell me a character secret. They kept coming back for the weird check and the fact the shadowy figure kept saying cryptic nothings.
If I was running this I would have the Nothics selling these secrets to your dungeons BBEG. I would give the player some secret disadvantage based on the number of secrets stolen which isn't relieved until that fight. Scatter them around the dungeon and make them a minion in that fight. I would make my BBEG something related to Vecna so an evil wizard. Allips would fit thematically as well