Alias, Lord of the Nine Hells

Discorporation. When someone possessing the divine heart of Asmodeus drops to 0 hit points or dies, his/her body is destroyed but the divine power and essence is returned to the heart.

Spellcasting. The Lord of the Nine Hells is a 9th level spellcaster. Its spellcasting ability is Intelligence (spell save DC 22, +12 to hit with spell attacks). The Lord of the Nine Hells has following spells prepared:
Cantrips (at will): [spell name], [spell name] 
5th level (5): Counterspell, Enervation                                                                                          

8th level (2): Abi Dalzum's Horrid Wilting

9th level (1): Storm of Vengeance

Manifest Echo. You can use a bonus action to magically manifest an echo of yourself in an unoccupied space you can see within 15 feet of you. This echo is a magical, translucent, gray image of you that lasts until it is destroyed, until you dismiss it as a bonus action, until you manifest another echo, or until you're incapacitated.

Your echo has AC 14 + your proficiency bonus, 1 hit point, and immunity to all conditions. If it has to make a saving throw, it uses your saving throw bonus for the roll. It is the same size as you, and it occupies its space. On your turn, you can mentally command the echo to move up to 30 feet in any direction (no action required). If your echo is ever more than 30 feet from you at the end of your turn, it is destroyed.

• As a bonus action, you can teleport, magically swapping places with your echo at a cost of 15 feet of your movement, regardless of the distance between the two of you.

• When you take the Attack action on your turn, any attack you make with that action can originate from your space or the echo's space. You make this choice for each attack.

• When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo's space.

Unleash Incarnation. You can heighten your echo's fury. Whenever you take the Attack action, you can make one additional melee attack from the echo's position.

You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain all expended uses when you finish a long rest.

Echo Avatar. You can temporarily transfer your consciousness to your echo. As an action, you can see through your echo's eyes and hear through its ears. During this time, you are deafened and blinded. You can sustain this effect for up to 10 minutes, and you can end it at any time (requires no action). While your echo is being used in this way, it can be up to 1,000 feet away from you without being destroyed.

Shadow Martyr You can make your echo throw itself in front of an attack directed at another creature that you can see. Before the attack roll is made, you can use your reaction to teleport the echo to an unoccupied space within 5 feet of the targeted creature. The attack roll that triggered the reaction is instead made against your echo.

Once you use this feature, you can't use it again until you finish a short or long rest.

Reclaim Potential. When an echo of yours is destroyed by taking damage, you can gain a number of temporary hit points equal to 2d6 + your Constitution modifier, provided you don't already have temporary hit points.

You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain all expended uses when you finish a long rest.

Legion of One. You can use a bonus action to create two echoes with your Manifest Echo feature, and these echoes can co-exist. If you try to create a third echo, the previous two echoes are destroyed. Anything you can do from one echo's position can be done from the other's instead.

In addition, when you roll initiative and have no uses of your Unleash Incarnation feature left, you regain one use of that feature.

Multiattack. The creature makes 4 attacks with its weapons

The Blade of the Ruined King. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: (4d6 + 10) slashing damage plus (2d4) necrotic damage.

Mace of the Black Crown. Melee Weapon Attack: +15 to hit, reach 10ft., one target. Hit: (3d6+10) plus (3d4) fire damage

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

Curse Divinity (Recharge 5 or 6). When a creature within 60 feet of the Lord of the Nine Hells makes an attack that deals radiant damage, He can teleport next to the attacking creature, and make one attack with The Blade of the Ruined King against the attacker. If the attack hits, the target is stunned until the end of its next turn.

Counterspell

Shadow of Moil (Costs 1 Action). Flame-like shadows wreathe your body until hit by an attack, causing you to become heavily obscured to others. The shadows turn dim light within 10 feet of you into darkness, and bright light in the same area to dim light. Until this spell ends, you have resistance to radiant damage. In addition, creatures have disadvantage on attack rolls against you, and whenever a creature within 10 feet of you hits you with an attack the shadows lash out at that creature, dealing it 2d8 necrotic damage

Melee Strike (Costs 1 Action). Make a melee attack with either The Blade of the Ruined King or The Mace of the Black Crown.

What Lies in the Dark (Cost 1 Action). Can swap places with any of the shadows created on the map.

Contract (Costs 2 Action). Create a temporary tether between you and another player (DC 20), until the end of the round, any damage done to you will be halved and dealt to them. On a save the tether is ended and the creature is immune for the next 24 hours.

Hell's Maw (Costs 3 Actions). Summon a gate to hell, a large skeletal head that opens its maw. The portal functions like a black hole, sucking any in range toward the opening. On a failed Dexterity save (DC 18), you begin to be pulled toward the portal at a speed of 60 ft per round. Those who pass will have half movement while moving away from or adjacent to the gate, within 100 feet of the gate. While going toward the gate, your speed is increased by 60. Magical means of transportation are not immune to this condition.

Daylight's End. You can choose one creature to make a DC 20 Wisdom Save, or be pulled into a plane of darkness for one round. The plane is not visible to others, and can be accessed through the ethereal plane. The creature may not leave until the end of the round.