Oh hey. I just got an idea for a mob. It's pretty rough though, so don't expect anything super great.
Basically, I imagined some kind of floating gas cloud in the shape of a cloaked figure. It would appear dark locations, and would actively avoid light. The monster would use 4 tentacle-like smoke appendages and grab onto the player's hitbox. The player would have to attack these appendages in around 5 seconds before at least 3 of them manage to latch on properly, after which the player would be dragged into the darkness. (The player can actively resist this.)
The monster would only take 20 or 30 percent of damage while in darkness, and while in darkness would appear much more solid. As for how it's actually defeated... I couldn't think of anything good. I just had "Creepy gas tentacle monster that dislikes light" and spewed that out here. Knowing you though, you'll think of something good.