As the entities that rank just below Hive Mothers and Elder Orbs in "Horrible things you never ever want to encounter", they are 100% awesome, and without giving anything away, I have placed them in a future dungeon for one of my lucky groups.
Anyway, I did a conversion when 5e was young, and reading it today, I felt it lacked any real punch ( a consequence I suspect of not really having a full feel for the system at that time). So, I have re-created this nightmare, giving it legendary actions, and making its eye powers a little more vicious. Xareth'Chelde by the way is the name for Beholders and their kind in my campaign world (Chelde is the "Mother of Horrors"; the nightmare god-thing some demented Hive Mother's call their Goddess, and Xareth means something along the lines of "Child of").
|"Tie a yellow ribbon, round the ole' oak tr-aaah-aaaaaaah-AAAAARGH!"|