Aetherglad

Regeneration. The Aetherglad regains 10 hit points at the start of its turn if it has at least 1 hit point, and is on the ethereal plane.

Magic Resistance. The Aetherglad has advantage against saving throws againts spells.

Spellcasting. The Aetherglad is an 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). The Aetherglad has following Sorcerer spells prepared:
Cantrips (at will): Thunderclap, Ray of Frost, Chill Touch, Frostbite 
1st level (4 slots): Magic Missle, Earth Tremor, Thunderwave

2nd level (3 slots): Blur, Hold Person

3rd level (3 slots): Counterspell, Dispel Magic, Vampiric Touch, Melf's Minute Meteors, Thunder Step

4th level (2 slots): Confusion, Banishment, Storm Sphere

Multiattack. The Aetherglad makes three attacks. One with its bite, one with its claw, and another bite attack.  The Aetherglad can replace any of its attacks with a cantrip. In exchange for two attacks it can begin its Arcane Drain.

Alternatively, the Aetherglad can cast a spell it knows, or use Ethereal Howl.

Bite.  Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 18 (2d10+7) piercing damage.

Ethereal Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., 1 target. Hit: 16 (2d6+7) cold damage and the target is grappled.

Arcane Drain. A grappled creature must succeed on a DC 17 Constitution saving throw or take 17 (4d6) psychic damage and lose access to the highest level spell slot until their next long rest on a failed save, or half as much and they retain their spell slots on a success. Every turn the creature is still grappled after activating this effect, they must make another saving throw or incur 7 (2d6) psychic damage and lose another level of spell. (passive after initial drain)

Frightful Presence. Each creature of the Aetherglad’s choice that is within 120 feet of the Aetherglad and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Aetherglad’s Frightful Presence for the next 24 hours. 

Ethereal Howl (Recharge 5-6). Each creature within 120 feet of the Aetherglad that can hear must succeed on a DC 17 Wisdom saving throw or take 66 (12d10) psychic damage on a failed save, or half as much on a successful one.

Blink. As a reaction to taking damage, the Aetherglad can teleport to an empty space up to 30 ft. away.

Legendary Resistance (3/Day). If the Aetherglad fails a saving throw, it can choose to succeed instead.

The Aetherglad can perform 3 legendary actions, choosing from the following options. Only one legendary action can be used at a time, and only at the end of another creature's turn. The Aetherglad recovers his legendary actions at the beginning of his turn.

One Attack Action.

Turn Etheral. Become resistant to all damage until end of next players turn.

Arcana Release (Costs 2 Actions). Release all Arcane energy absorbed in one large blast. Every creature within 60 feet must succeed on a DC 17 Dexterity saving throw. For each successful use of Arcane Drain, the damage is another 2d12 of force damage on a failed save, or half as much on a successful one.