Agares

Devil's Sight. Magical darkness doesn't impede the creature's darkvision.

Divine Awareness. The creature knows if it hears a lie.

Hellish Weapons. The creature's weapon attacks are magical and deal an extra (2d8) poison damage on a hit.

Spellcasting. Agares is a 19th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +2 to hit with spell attacks). Agares has following Sorcerer spells prepared:

Cantrips (at will): firebolt, mind sliver, minor illusion, true strike

1st level (4 slots): burning hands, chaos bolt, false life, sword burst

2nd level (4 slots): crown of madness, detect thoughts, shadow blade, wither and bloom

3rd level (3 slots): conjure lesser demon, fear, incite greed

4th level (3 slots): confusion, spirit of death, wall of fire

5th level (2 slots): cloudkill, insect plague, telekenesis

6th level (2 slots): circle of death, mental prison, scatter

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

8th level (1 slot): Abi-Dalzim's horrid wilting, earthquake

9th level (1 slot): blade of disaster, psychic scream

Darkness Aura (1/day). A 15-foot radius of magical darkness extends out from the monster, moves with it, and spreads around corners. The darkness lasts as long as the monster maintains concentration, up to 10 minutes (as if concentrating on a spell). Darkvision can't penetrate this darkness, and no natural light can illuminate it. If any of the darkness overlaps with an area of light created by a spell of 2nd level or lower, the spell creating the light is dispelled.  

Hellfire Orb (1/day). The monster hurls a magical ball of fire that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot radius sphere centered on that point must make a DC 18 Dexterity saving throw. The sphere spreads around corners. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Mind Blast (1 per 5 turns). The monster magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Decaying Blade. Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 24 (3d12 + 2) necrotic damage. 

Counterspell. You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a success, the creature’s spell fails and has no effect. At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.

Shield (2nd level, 1 per 4 turns). When Agares is hit by an attack or targeted by magic missile, it gains a +5 bonus to AC (including against the triggering attack) and immunity to magic missile. These benefits last until the beginning of its next turn. 

Agares can take 3 legendary actions per turn, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Agares regains spent legendary actions at the start of its turn. 

Frightening Gaze (costs 2 actions). The monster fixes its gaze on one creature it can see within 10 feet of it. The target must succeed on a DC 18 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the monster's gaze for the next 24 hours.

Summon Soul (costs 1 Actions). Once per turn, the monster can summon a soul that it has captured. This soul can use one attack (melee or ranged) against the target, and the target will take 4d6 damage. After the soul attacks, it immediately disappears. 

Vile Curse (costs 3 actions). The monster targets one creature it can see within 30 feet of it. The target must succeed on a DC 15 Wisdom saving throw or be magically cursed. Until the curse ends, the target has disadvantage on attack rolls and saving throws. The target can repeat the saving throw at the end of each of its turns, ending the curse on a success.

Read the follow text when Agares' mythic trait is activated:

Shadows begin to swirl around Agares' fallen form, his aura pulsing. The pulsing quickens until the darkness surrounding him blasts out from his body, and you are thrown backwards. As you sit up, you see piercing red eyes are glaring down at you through a shroud of darkness. As the shadow form changes and morphs, you are horrified to see that Agares has changed into a Demonic Dragon, poised to attack. Prepare yourself!

Demonic Form (mythic trait, recharges after long rest). When Agares reaches 0 hit points, he doesn't die or fall unconscious. Instead, the last scrap of his existence siphons the energy from his stolen souls, and he bursts with necrotic energy to regain 400 hit points. 

If Agares' Demonic Form is active, it can ONLY use the options below in battle:

Negative Energy Blast (1 per 2 turns). Every creature inside the Negative Energy Cone must succeed on a DC 17 Dexterity saving throw, taking 42 necrotic damage on a failed save, or half as much damage on a successful save. 

Withering Fire Breath (1 per 5 turns). Every creature within a 120 ft cone must succeed on a DC 16 Constitution saving throw, taking 50 fire and necrotic damage on a failed save, or half as much damage on a successful save. On a failed save, the creature(s) will also continue to take 2d10 damage at the start of each of its next 3 turns. 

Show of Force. The Demonic Dragon uses its bite and tail attack, dealing 4d10 damage on a successful hit.