Anesthesion

Amphibious. Anesthesion can breathe air and water.

Spell Mastery. Anesthesion can cast the 1st level spell Shield and the 2nd level spell Ray of Enfeeblement at will without expending a spell slot. Anesthesion must expand a spell slot as normal if cast at a higher level.

Sculpted Spells. Anesthesion  can create pockets of relative safety within the effects of his evocation spells. When he casts an evocation spell that affects other creatures that he can see, he can choose a  a number of them equal to 1 + the spell's level. The chosen creatures automatically succeed on their saving throws against the spell, and they take no damage if they would normally take half damage on a successful save.

Over-channel. When Anesthesion casts a wizard spell of 1st through 5th level that deals damage, he can deal maximum damage with that spell. The first time he does so, he suffers no adverse effect. If he uses this feature again before he finishes a long rest, he takes 2d12 necrotic damage. This goes up by 1d12 for each time used. The necrotic damage ignores resistance and immunity.

Spellcasting. Anesthesion is a 19-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks, and adds a +7 to all damage roll). Anesthesion has following Wizard spells prepared:
5 Cantrips (at will): Fire Bolt, Shocking Grasp,  Poison Spray, Ray of Frost, Acid Splash
1st level (4 slots): Burning hands, Magic missile, Mage Armor, Thunderwave

2st level (3 slots): Enlarge/Reduce, See Invisibility

3st level (3 slots): Lightning Bolt, Fireball, Counterspell,

4st level (3 slots): Banishment, Dimension Door, Otiluke’s Resilient Sphere, Wall of Fire

5st level (3 slots): Cone of Cold, Wall of Force, Wall of Stone

6st level (2 slots): Globe of Invulnerability, Otto’s Irresistible Dance

7st level (1 slots): Teleport

8st level (1 slots): Antimagic Field

9st level (1 slots): True Polymorph, Time Stop, Foresight

Multiattack. Anesthesion makes two Lightning Sword attacks or uses Wind Javelin twice.

Lightning Sword. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 40 (9d6 + 9) lightning damage. 

Wind Javelin. Anesthesion coalesces wind into a javelin-like form and hurls it at a creature it can see within 600 feet of it. The javelin is considered a magic weapon and deals 19 (3d6 + 9) piercing damage to the target, striking unerringly. The javelin disappears after it hits.

Anesthesion can take 3 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 creature's turn. Anesthesion regains spent legendary actions at the start of his turn.

Gust. Anesthesion targets a creature it can see within 60 feet of it a creates a magical gust around it. The target must succeed on a DC 18 Strength saving throw or be pushed up to 20 feet in any horizontal direction Anesthesion chooses.

Thunderbolt (2 Actions). Anesthesionhurls a thunderboltat a creature it can see within 600 feet of him. The target must make a DC 18 Dexterity saving throw, taking 22(4d10) thunder damage on a failed save, or half as much damage on a successful one.