Aero Siin

Tiamat's Investment. At the start of Aero's turn, roll a d6. On a roll of 5-6 Aero get's back all his spell slots.

Undying Slave: Twice, when Aero's Hit Points reach 0, he regains his full health.

Spellcasting. Aero is a 9th-level spellcaster. His spellcasting ability is [Charisma] (spell save DC 17, +9 to hit with spell attacks). Aero has following Warlock spells prepared:
Cantrips (at will): Eldritch Blast, Blade Ward, Minor Illusion 
5th level (4 slots): Hellish Rebuke, Darkness, Mirror Image, Invisibility, Misty Step, Dimension Door, Spirit Shroud, Shadow of Moil, Summon Greater Demon*

Chain Master Spell. Spells with a range of touch, self, or 5ft. can be cast through a familiar using the familiar's reaction.

*Summons Beldr

Multiattack. Aero can attack twice, instead of once, whenever he takes the Attack action on his turn. Moreover, he can cast one of his cantrips in place of one of those attacks.

Scimitar of Speed. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: # (1d6 + 4) [Piercing] damage. 

Eldritch Blast Ranged Magic Attack: +9 to hit, range 120ft.,  2 beams. Hit: # (1d10 + 5) [Force] damage.

Chain Master: Attack

Hellish Rebuke.

Chain Master- Protect. When your familiar takes damage, you can use your reaction to grant it resistance against that damage.

Aero had 2 legendary actions that he can take at any time during another creature's turn. He regains both actions at the start of his turn.

Blade Ward (1 Action). Casts the Blade Ward cantrip.

Misty Step (2 Actions). Casts the Misty Step spell, using 1 warlock spell slot. Can not be used if Aero has no spell slots remaining.

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

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