For statblocks my go to are ones made by Conflux Creatures: Nothics, including some bits of lore and tactics.

The temple would surely have some Animated Objects as well!

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

...sounds like a great opportunity to mash-up allip features for extra-spicy nothics...

A nothic that has discovered a ring of invisibility and a ring of mind shielding. It lives in the walls of the king's palace, and wears the rings to listen in on war meetings and diplomatic visits.

The king has no idea how state secrets keep escaping his confidence, and he's begun to turn paranoid and anxious, firing and exiling advisors left and right. The nothic is taking advantage of this, and is gleefully driving an entire nation into ruin and revelling in the fact that this secret belongs to it and it alone.

I used the nothic in phendelver. My players were intrigued and terrified with a green flavoured mind monster.

If you had a city of them, I'd imagine you'd have this cacophony of mental discussions. Kind of like being in a crowd, but noone being around. This could create some healthy speculation amongst the group. Maybe you could make some of the players more sensitive to the talk giving them lore, but a disadvantage if it gets too loud.

Darkness was the main pull of the lair in phendelver, so I'd lean on that. Lots of hiding spots and mind speak makes the monsters seem hard to find which adds tension.

Let us know how you do.