Spellcasting. Aclazotz is a 18th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 20, +13 to hit with spell attacks). Aclazotz has following Wizard spells prepared:
Cantrips (at will): mage hand, prestidigitation, ray of frost 

1st level (5 slots): detect magic, magic missile, shield, thunderwave

2nd level (4 slots): detect thoughts, invisibility, Melf’s acid arrow, mirror image

3rd level (4 slots): animate dead, counterspell, dispel magic, fireball

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

5th level (4 slots): cloudkill, scrying

6th level (2 slots): disintegrate, globe of invulnerability

7th level (2 slots): finger of death, plane shift

8th level (2 slots): dominate monster, power word stun

9th level (1 slot): power word kill

Bite.  Melee Weapon Attack: +20 to hit, reach 15 ft., one target. Hit: 40 (5d20 + 5) piercing damage.

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 25 (3d10 + 5) slashing damage 

Life Drain. RangedWeapon Attack: +16 to hit, range 120ft., 4 target. Hit: 20 (3d20 + 5) necrotic damage.

Frightful Presenc. Each creature of Aclazotz’s choice within 120 feet of it and aware must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if Aclazotz is within sight, 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 Aclazotz’s Frightful Presence for the next 24 hours.

Soul Charge. When Aclazotz kills or downs an opponent he can regains 10hp

Death orb. If Aclazotz would be hit by a critical it takes normal damage

Aclazotz can make 1 legendary action. Aclazotz regains spent legendary actions at the start of his turn.

True Fear (Costs 1 Actions). Summon 3 Giant bats

Aclazotz has 2 mythic actions which are regained after long rest

Just Cruelty. (Costs 2 Actions). If Aclazotz rolls a critical hit the opponent takes an additional 2d4 necrotic damage 

Gust of pain. (Cost 1 action) Aclazotz flaps his wings sending all 15m away and dealing 3d6 force damage