Anaxagorath

Action surge: Another action (twice per long rest)

indominable: you can reroll a saving throw (3 times per long rest)

Greatweapon master: -5 to hit + 10 to damage.

 Ring of spell turning: While wearing this ring, you have advantage on saving throws against any spell that targets only you (not in an area of effect). In addition, if you roll a 20 for the save and the spell is 7th level or lower, the spell has no effect on you and instead targets the caster, using the slot level, spell save DC, attack bonus, and spellcasting ability of the caster.

Deck of Many Things:

Chosen of the outer deities: when you are taken down to 0 HP, you regain 480HP and Summon 3 dark blades, a pair of black wings with purple eyes grow from your shoulders, gain a flying speed of 60 feet and you have true sight to 300 feet, also all legendary resistances are restored. While within this state Anaxagorath personality is replaced with Yog-Sothoth. this ability does not recharge naturally in order to regain their normal form it requires a ritual that takes 1 week. 

Action Name. Enter the description for your action.

Multiattack. Anaxagorath can make five attacks

Action Melee Attack. Moonblade: +16 to hit, reach 10 ft., 1 target. Hit: 21 (3d6 + 12) [slashing] damage. Crits on an 18, contains dimension door. On a crit deal an extra 2d10 damage and heal that amount.

Action Ranged Attack. RangedWeapon Attack: +17 to hit, range 150/600 ft., 1 target. Hit:  22 (2d8 + 13) [damage type] damage.

Bonus Action: Summon dark blade: Anaxagorath can summon 2 dark blades, these blades can be used to attack +13 to hit, reach 10 ft., 1 target. Hit: 16 (2d6 + 10) [slashing] damage an as a bonus action , while at least 1 sword is summoned  Anaxagorath can summon 1 additional sword, as part of this bonus action all swords can move 30 feet , the swords have 20hp and an AC of 18, they are immune to all damage other than radient, slashing, bludgeoning, piercing. Each blade within 5 feet gives +1 AC

Rebuking strike: when attacked Anaxagorath can increase his AC by 2, if this causes the attack to miss he can create another dark blade within 30 feet, which can immediately make 1 attack.

Teleporting blast: If Anaxagorath is hit by an attack, he can teleport up to 20 feet away and create a blast of force, each creature within 20 feet must make a con save (DC 18) or be forced back 15 feet and take 3d6 force damage.

Anaxagorath has 3 Legendary actions, 

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.

reactive: gives another reaction

Sword strike: Can Teleport to a dark sword within 20 feet and make one attack with it. 

Mind break: You hold out your hand, the eldritch entity inside you lets out a psychic howl at a creature you can see within 60 feet, the must make an intelligence save DC 18. On a failed save, the creature is stunned till it's next turn. Recharges on a 4, 5 or 6..

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