Magic Resistance. Nadriel has advantage on saving throws against spells and other magical effects.

Legendary Resistance (5/day). If Nadriel fails a saving throw he can choose to succeed instead.

Spellcasting. Nadriel is a 20th level spellcaster. His spellcasting ability is Intelligence (spell save DC 27, +19 to hit with spell attacks). Nadriel has the following spells prepared:

• Cantrips (at will): fire bolt, mage hand, message, shape water, mind sliver, sapping sting, mage hand, shocking grasp
• 1st level (at will): shield, absorb elements, charm person, disguise self, sleep, gift of alacrity
• 2nd level (at will): alter self, blur, detect thoughts, invisibility, mirror image, suggestion, mind spike, shadow blade, tasha's mind whip
• 3rd level (at will): spirit shroud, counterspell, dispel magic, fireball, hypnotic pattern, lightning bolt, pulse wave, slow, haste, sending
• 4th level (4 slots): charm monster, summon aberration, banishment, blight, confusion, dimension door, gravity sinkhole, arcane eye
• 5th level (4 slots): synaptic static, bigby's hand, dominate person, dream, geas, modify memory, wall of force, teleportation circle, temporal shunt
• 6th level (3 slots): mental prison, gravity fissure, arcane gate, chain lightning, contingency, disintegrate, mass suggestion
• 7th level (3 slots): tether essence, power word pain, forcecage, prismatic spray, simulacrum, reverse gravity, teleport, delayed bast fireball, finger of death, plane shift, sequester
• 8th level (2 slots): dark star, reality break, maddening darkness, demiplane, dominate monster, feeblemind, mind blank, power word stun, clone maze
• 9th level (2 slots): meteor swarm, blade of disaster, psychic scream, foresight, gate, power word kill, time stop, wish, ravenous void, time ravage

Infallible Insight. Nadriel knows when he hears a lie.

Eldritch Mastery. Nadriel can maintain concentration on two spells at once. If he takes damage he makes separate concentration checks on each of the spells. Furthermore, Nadriel has advantage on constitution saving throws made to maintain concentration on spells.

Psychic Maelstrom. Some of Nadriel's abilities cause creatures to have psychic energy surrounding them. A creature should keep a tally of this psychic energy. Whenever Nadriel deals psychic damage to a creature they take an additional 1d6 psychic damage per tally they have of psychic maelstrom then they gain a tally of psychic maelstrom. At the start of each creatures turn, if that creature has 1 or more tally's of psychic maelstrom then they can make an intelligence saving throw. Subtract 15 from the total and divide by 5 (rounding down) the total value is the number of tally's that the creature loses.

Arcane Wards. Nadriel adds a bonus to his AC equal to half his intelligence modifier

Multiattack. Nadriel makes two Malevolence, Piercer of the Immortal attacks. He can replace one of these attacks with a tentacles attack or an extract brain attack.

Malevolence, Piercer of the Immortal. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 12 (1d10 + 7) piercing damage and 28(8d6) psychic damage. 

Tentacles. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 21 (2d10 + 10) psychic damage. If the target is a creature which is medium or smaller it is grappled and must succeed on a DC 27 intelligence saving throw or be stunned until the grapple ends. 

Extract Brain. Melee Weapon Attack: +19 to hit, reach 5 ft., one incapacitated target who is grappled by Nadriel. Hit: 55 (10d10) piercing damage. If the target is reduced to 0 hit points it dies as Nadriel devours its brain. In this case Nadriel gains 40 temporary hit points. 

Teleport. Nadriel teleports up to 130ft to an unoccupied space that he can see2

Nadriel can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creatures turn. Nadriel regains spent legendary actions at the start of his turn.

Teleport. Nadriel uses his teleport bonus action.

Attack (Costs 2 actions). Nadriel makes a melee attack

Cantrip (Costs 2 actions). Nadriel casts a cantrip from his spell list

Spell (Costs 3 actions). Nadriel casts an at will spell from his spell list

Greater Spell (Costs 4 actions). Nadriel casts a spell from his spell list expending a spell slot as normal

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

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.