Alsio the Archmage

Amphibious. Alsio can breath both air and water.

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

Spellcasting. Alsio is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). He can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): mage hand, prestidigitation, ray of frost

1st level (4 slots): detect magic, magic missile, shield, thunderwave

2nd level (3 slots): acid arrow, detect thoughts, invisibility, mirror image

3rd level (3 slots): animate dead, counterspell, dispel magic, fireball

4th level (3 slots): blight, dimension door

5th level (3 slots): cloudkill, scrying

6th level (1 slot): disintegrate, globe of invulnerability

7th level (1 slot): finger of death, plane shift

8th level (1 slot): dominate monster, power word stun

9th level (1 slot): power word kill

Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Desiccating Breath (Recharge 5–6). Alsio exhales yellowish necrotic energy in a 15-foot cone. Each creature in that area must make a DC 11 Constitution saving throw, taking 21 (6d6) necrotic damage on a failed save and is weakened until the end of its next turn. A weakened creature had disadvantage on Strength-based ability checks and Strength savings throws, and the creature's weapon attacks that rely on Strength deal half damage. On a successful save, the creature takes half as much damage and isn't weakened.