Archduke Dean: Lord of Avernus

Legendary Resistance( 3/day)- if dean fails a saving throw he can choose to pass it

Magic Resistance- dean has adv on saving throws against spells and other magical effects 

Properties of the Eye. Your alignment changes to neutral evil, and you gain the following benefits: You have truesight. The eye has 8 charges. You can use an action and expend 1 or more charges to cast one of the following spells (save DC 18) from it: clairvoyance (2 charges), crown of madness (1 charge), disintegrate (4 charges), dominate monster (5 charges), or eyebite (4 charges). The eye regains 1d4 + 4 expended charges daily at dawn. Each time you cast a spell from the eye, there is a 5 percent chance that Vecna tears your soul your body, devours it, and then takes control of the body like a puppet. If that happens, you become an NPC under the DM’s control.

Properties of the Hand: Your Strength score becomes 20, unless it is already 20 or higher. Any melee spell attack you make with the hand, and any melee weapon attack made with a weapon held by it, deals an extra 2d8 cold damage on a hit. The hand has 8 charges. You can use an action and expend 1 or more charges to cast one of the following spells (save DC 18) from it: finger of death (5 charges), sleep (1 charge), slow (2 charges), or teleport (3 charges). The hand regains 1d4 + 4 expended each daily at dawn. Each time you cast a spell from the hand, it casts the suggestion spell on you (save DC 18), demanding that you commit an evil act. The might have a specific act in mind or leave it up to you.

Properties of the Eye and Hand. If you are attuned to both the hand and eye, you gain the following additional benefits: You are immune to disease and poison. Using the eye’s X-ray vision never causes you to suffer exhaustion. You experience premonitions of danger and, unless you are incapacitated, can’t be surprised. If you start your turn with at least 1 hit point, you regain 1d10 hit points. If a creature has a skeleton, you can attempt to turn its bones to jelly with a touch of the Hand of Vecna. You can do so by using an action to make a melee attack against a creature you can reach, using your choice of your melee attack bonus for weapons or spells. On a hit, the target must succeed on a DC 18 Constitution saving throw or drop to 0 hit points. You can use an action to cast wish. This property can’t be used again until 30 days have passed.

Spellcasting. Dean is a 20 level spellcaster. Its spellcasting ability is [Int] (spell save DC 24 [Dean] has following [Wizard] spells prepared:
can- blade ward, sword burst

lvl 1- shield, sleep, long strider

lvl 2-blindness/deafness, darkness, hold person 

lvl 3- spirit shoud, fire ball, counter spell

lvl 4- Dimension Door, polymorph, banishment

lvl 5- dominate person, contact other plane, steel wind strike

lvl 6- tensers transformation, mental prison

 lvl 7-  tether essence,  mordenkainens sword

 lvl 8- feeblemind

lvl 9- Blade of Disaster

Yamato. Melee Weapon Attack: +16 to hit, reach 5 ft., 1 target. (2d8 + 2d8 + 7) [Slashing/Cold] damage. 

Beowulf: Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target (3d4 + 2d8 + 5) [Bludgeoning/Cold] damage. 

Force Edge: Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target (3d6 + 2d8 + 5) [Piercing/Cold] damage. 

Judgment Cut:  Ranged Weapon Attack: +16 to hit, range 80/300 ft., 1 target.(2d12+7) [slashing] damage.(if you roll a 18,19 or 20 you get to do another judgment cut for free)

Multi Attack: Make four attacks using Yamato, Beowulf, Force Edge or judgment cut in any order

Show me your motivation: You can use an action to see as if you were wearing a ring of X-ray vision. You can end this effect as a bonus action.

Teleport- dean magically teleports, along with any equipment she is wearing and carrying, up to 120 feet to an unoccupied space she can see

Plain shift (3 Charges)- You and up to eight willing creatures who link hands in a circle are transported to a different plane of existence. You can specify a target destination in general terms,  you appear in or near that destination GM’s discretion.
You can use this spell to banish an unwilling creature to another plane. Choose a creature within your reach and make a melee spell attack against it. On a hit, the creature must make a Charisma saving throw. If the creature fails this save, it is transported to a random location on the plane of existence you specify. A creature so transported must find its own way back to your current plane of existence.

Enter the description for how bonus actions work for your monster here.

Don't get cocky- as a reaction, stop one melee atk that was within 5 feet from you and get a attack of opportunity with the Yamato.

Slay All..(Judgment Cut End):(2 actions) charging up a atk with the Yamato, all creatures in a 40 ft. radius must make a Dex saving throw of 18 or higher or take 10d10 force dmg of half if you passed

Prepare... to DIE!.:(2 actions) charging a attack with Beowulf, all creatures in a 80 ft. radius must make a Str saving throw of 18 or higher or take 8 d8 thunder dmg and be knocked prone. if you pass only take half dmg 

Teleport: (1 action) use his teleport action

Your nightmare begins here!- when dean gets to a least half health go into your sin devil trigger: Increase movement speed by 20 feet and gain a 90ft flying speed, Ac increases to 23 and add a additional 20 radiant to all melee atks.

How droll... that's enough!: summon a energy doppelganger every time dean loses 100 hp (there can only be one doppelganger on the field, ), the doppelganger  has 80 hp and has an AC of 17. it has extra attack, it has speed equal to deans current moment speed