So in my upcoming session, 7 level 10 players are entering a dungeon within a massive library. This library is known to keep dangerous magic items and forbidden knowledge. The section they are entering is a highly restricted section. They will be able to enter through the door to start but inside is meant to be a dungeon/ labyrinth. Guarding this is some sort of monster called "The Silent Guardian". I am looking to have this entire encounter be based around how quiet they can be. Their goal is to find this magic item they need and a magic scroll, where both of these items are located in random parts of the dungeon library. The Silent Guardian sleeps until they hear a distrubance. This will be made on a perception check and it would be rolled if someone casts a spell requiring words, or if a player uses their weapon in a destructive manner. Their goal is get their items while avoiding the guardian, or fight it to the death. This is meant to be a very deadly encounter if they come across the guardian so avoiding it may be in their best interest.

The dungeon would be entirely dark with lots of smaller unknown creatures running around. So it is possible for them to run into said creatures and have to fight them off. If they are too loud, then they could attract the attention of the guardian. Perhaps these creatures are the minions to the guardian in some way? Anyway, the guardian would be blind but can now use its own hearing to keep their library silent. This guardian could be intelligent but they are blind because one of the players in the party had blinded it during a fight in their backstory. I'm going off of the guardian not knowing who they are as long as the player doesn't do something to indicate they were the ones who blinded them long ago. So it is possible for them to negotiate with the guardian but they would view any and all adventurers as threats since it was one who blinded them long ago.

I am still doing research in a good monster to use in this but I can't think of anything or find anything at the moment. I'm not sure what would be good here and I don't want to homebrew as I am terrible at doing that kind of thing. When I have run the homebrewed monsters I made before, they were terribly unbalanced and made the session go bad. I'm not opposed to using someone else's homebrewed monsters but I'm still hesitant towards it. So if someone knows a good official dnd monster that may need a few tweaks, I am all for it. I appreciate your help and advice! I welcome any other ideas too that could make this an even more awesome encounter. 

Have you looked at the Archaic from Strixhaven.  I think it has the right flavor and it has some neat tricks (I think Gravity Shift would work really well in a library and Spell Mimicry is a great surprise).  It's also pretty obscure so most players won't have encountered it. 

There are a few things to consider tweaking, though -

• It has Truesight so you might need to change this to blindsight and reduce the radius a lot (down to 20ft for the stealth aspect) to better suit your story (you can't give him the Blinded condition as this would make the encounter way too easy and stop him from using some spells). 
• Its CR is 18 so this might make it a little easier for your large group especially if you use a lot of ranged attacks that don't rely heavily on magic but, hey, figuring out the trick is what makes boss battles fun. That said, adding any type of minion to split the party's focus could turn this encounter deadly. 
• It's also gargantuan so it looks intimidating but might be too big for your library (although, if your party was unprepared, this might work in your favor, as they can flee down a small tunnel, etc. and give the Archaic's alertness a chance to reset a la Metal Gear Solid - he wouldn't damage his precious books unless he had discovered that the player was the one who had done this to him).  Again, it is easy enough to just reduce the size to large without changing any of his other stats. 
• You might also consider nerfing his perception, as it's pretty high.

As a plot point, maybe one of your magic users was researching reincarnation spells and accidentally sent another scholar (perhaps it was a good friend or, even more angering, a random student the player didn't know existed and didn't notice he had vanished) back in time to be reincarnated as the Archaic, obviously as he was not ready this could account for the changes to his stats, rather than just being blinded (also, I imagine he'd be pretty annoyed and confused after living through millennia all at once and not being prepared - his only anchor being the library he once studied in for hours, everyday).  Having a scholar recognizing it as an Archaic but defective could add to the intrigue.  A crazy, omniscient celestial with a grudge sounds like some fun interactions if they get the chance.

Anyway, if you make the above changes, I think you'll have a pretty balanced encounter that would be a challenging battle, an interesting stealth mission or a really interesting role-play.

If we’re not looking at the very obvious (but very powerful) Miirym (from Candlekeep Mysteries) I might actually suggest an Eidolon or two.  Place Sacred Statues liberally throughout the Library and this becomes a possibly unending chain of combat encounters, where every time the party destroys one Statue, the Eidolon flits away to possess another Statue for pursuit.  Make it two Eidolons and things get very serious very fast.  And these guys hit HARD.  Perception for the Eidolon isn’t that great at +8, so it certainly could be Stealthed past if the party took things seriously.

Obviously I also suggest 2 Iron Golems (mildly upgraded (mostly in hit points, but a bit of a damage boost would also be good) or perhaps just a single but strongly upgraded one.  Forget that.  I actually recommend 2 Canopic Golems as these guys are actually tougher and thematically appropriate IMO.

If you do decide to go with Miirym, I demand to get a post battle recap from you.  Be aware that the one extreme negative about Miirym is their +16 (!!) to Perception.  You would probably need to nerf that.

For some reason, I get a vibe that the Silent Guard is likely a construct that is either highly resistant to most forms of magic, or outright, immune? Perhaps the players have to use their wits about them and discover that it either has a weak spot, or there is a certain environmental hazard that they can use to either weaken it, or immobilize it? Perhaps its undefeatable in the traditional sense?

A highly modified Cogwork Archivist could work? I'd feel that to be quite fitting. Or maybe something more crude, like a mutilated Beholder, where its mouth is sewn shut, being a literal, Silent Guardian.

Regardless of what the big monster is, please please include a Nothic somewhere as an encounter - and not necessarily combat! This is one of the few opportunities that are perfect to add one, and they are such cool monsters with lore that is very fun to play with. 

Thank you! These will all be really helpful! It's gonna be a multi room dungeon that could be a 2 session thing. I really appreciate all if the suggestions and will be using many of them. 😁

You could also use magic books as traps -- touch it and you get sucked into the story and have to resolve whatever scene you're in.