This seems so overpowered

So, I have a few critiques. I like the idea, but I feel like it can be better improved to become something that players can actually combat, and might fit better into the world it is placed in. Before I go into any detail for the Spells, Actions, and Legendary Actions, I'd like to mention the ranges you are using, as they are a bit different. 5th Edition is mostly built around ranges based on 30 ft. Granted there are exceptions, but this pseudo-rule effectively creates a balance for spellcasting. Giving a creature spells with a range of 40 ft allows them to attack outside of range of the 30 ft spells. This isn't a problem on it's own, but it should be considered when giving a creature unique ranges.

First off let's talk spells. Every single spell is combat oriented, which is tough because it means most of to them will never have any use on the battlefield because fights in 5th Edition are not really meant to last past 6-7 rounds. I'd recommend swapping some spells to be non-combat utility based. For example, Disguise Self is a simple spell with many uses. I'd also like to comment on the Arcane Mastery Legendary Action. 4 times a day means that it can cast 10 7th-level spells or 5 9th-level spells in one day, which is insane. I'd restrict it to maybe a 1/day feat, but probably more of a 1/week ability so that players can force the creature to use it, then back off and fight it again later, but I'll get to Legendary Actions later.

So, next up are the Actions, and these aren't the worst. First thing to mention is the  lack of Multiattack. Multiattack is a pretty vital tool for most monster, especially powerful boss monster. In a round of 5th Edition, the players will likely have the most amount of actions when fighting a single creature, which is why Multiattack is a feature. It allows single creature combat to be more than just wailing on a monster that will hit you once every turn. Next is the Dagger of Petrifying. At first I wasn't a fan, but after re-reading it I think it could be a unique weapon, I think you might want to gauge player reactions to it and alter it afterwards to reflect player feedback. The Throwing Disk attack is perfectly fine, but the Glare from Beyond the Grave is brutal. Fear gives some nasty effects to those affected by it, and the 5 minute timer is just brutal considering that 10 round of combat in 5th Edition is equal to a minute of passing time. You are going to want to consider putting a way to removing that fear effect into the ability or it will cause some serious problems.

Final we get to the Legendary Actions, and here is where I think we have some really great ideas that could use some minor tweaks. First, you have no 1 Action abilities, meaning the creature will only get one Legendary Action a round. I'd fix this by changing the Backstabbing action. I would have the attack not use the Petrifying dagger because you can easily have a TPK with him flying around attacking with advantage with that, so maybe he has a Dagger of Teleportation type weapon that grants him this backstab ability. Then decrease the cost of the ability to 1 Legendary Action and you would likely be in a better place. Next is the Madness in a Box ability. Forcecage is a spell where if you can deal with it, you are out of the fight. No one has Disintigrate or a way of teleporting out of the cage? Tough luck, you aren't in this fight anymore. I'd remove this ability and replace it with a movement ability or a regular Cast a Spell ability. Legendary Action movement without provoking opportunity attacks is an incredibly usely ability, as is the ability to cast one of your prepared spells. And I have already mentioned the Arcane Mastery ability, but I'll bring it up one more time. I truly believe this is the most "overpowered" feature of this creature. It sounds like a cool ability, but it either needs to only function on lower level spells similar to the Wizard's Arcane Recovery feature, or have way less uses, potentially even only having a one week cooldown on the ability.

To end this note, I'd just like to once again reiterate, I think this is a cool monster. I just feel that it was made strictly to kill characters. Character death can be an amazing event, full of emotions of a friend falling valiantly in battle, but it can also be an unsatisfying campaign ruining moment. It all depends on how much agency the players had over their death. Power Word Kill will just kill you, Forcecage will just make you be out of a fight because it doesn't take concentration. These things can be an intense moment of problem solving, or they can cause all your players to become very upset at you for just killing them off without giving them a fighting chance. DnD is a game after all, and unless you have talked to your players about it before your first session, an unwinnable fight will leave a bad taste in everyone's mouth.

If you are interested in running this monster, as it does have a truckload of potential, I posted my proposed tweaks for it as a different comment.

Wow Jericho you must have analyzed this so much I’m lovin it.

dito