Archaemon

Special Trait. Demonic Menacing - Any creature Archaemon deems an enemy is intimidated by it's demonic presence. The target(s) must roll a Wisdom check DC25 or have disadvantage on any attack, spell or save made against it. Once every two turns all creatures, affected or not, had to reroll to see whether they are still or become effected

Demon Lord's Strength : Crits may be scored in a 18 and 19 as well as 20. 

Spellcasting. Archaemon is a Max-Level spellcaster. Its spellcasting ability is [ability score] (spell save DC 25, +6 to hit with spell attacks). The [monster name] has following [class name] spells prepared:
Cantrips* (at will): [Eldritch Blast], [Spirit Guardians], [See Invisibility], [Darkness], [Misty Step]

*Archaemon is the embodiment of all evil magic, so treats all spells as a cantrip due to the ability to cast at will.

Action Name. Multiattack: Archaemon can choose any combination of attacks and spells, totalling 4/turn

Action Melee Attack. Archaxe: +10 to hit, reach 15ft., 1 target. Hit: (4d10 + 10) [Slashing] damage. 

Action Ranged Attack. Eldritch Blast: A beam of crackling energy streaks toward a creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 force damage.

The spell creates more than one beam when you reach higher levels: Normally, a warlock gets a maximum of 4 beams, but as Archaemon is the embodiment of evil magic, he is able to create 5 beams at a time. He also, similarly to a warlock, add his Charisma modifier to the damage delt each beam.

Archaemon gets 2 bonus actions per turn, used for:

- Demonic Frenzy - using one bonus action, Archaemon sends himself into a rampage, gaining advantage on every roll. To sustain this frenzy, at the end of every subsequent turn he must forgo his 2 bonus actions. If he has already used one or both bonus actions, the frenzy ends then{1}, or it ends if he chooses to with one bonus action{2}. If the frenzy ends with {1}, then all rolls against him have advantage and the Demonic Menacing effect is temporarily halted, resuming where it left off his next turn. If it ends with {2}, then all rolls on him have advantage for 4 turns, and he cannot act the first of those turns, and the Demonic Menacing effect is temporarily halted for 3 of those turns, resuming where it left off his next turn.

Demonic Second Wind: Only Usable Twice: recovers 10 D20 hitpoints.

Demonic Blast: Blast everything away in a Sphere with a 25ft radius, dealing 4d6 damage and knocking a creature 20ft back and prone, and half as much damage, half as far and not prone on a successful save (CON DC20)

Demon lord's Luck: When rolling a D20, 1 bonus action may be used to reroll any die that rolled a 5 or less. This may be used with both bonus actions.

Reaction Name. Hellish Rebuke II: When hit by an attack from up to 80ft away, Archaemon can point his finger at the target and a mass of hellish energy burts from out the ground, dealing 4d6 on a failed Dex save (DC 20) and half as much on a successful save. On a failed save, the target is also knocked prone 

Archaemon gets 2 legendary actions, each with a separate 4 turn cooldown. These can be used between Anyone's turns.

Legendary Action Names (Costs 1 Action each). 

Demonic Bonus - Used as any extra Action, Bonus action or re-action {without needing to be attacked}

Death's Boss - Deals 50% max a creature's health to it in damage.

None (It Isn't Mythic)