Ashiok's Nightmare Amalgam

Anomalous Anatomy. The Nightmare Amalgam does not need to eat or breath. 

Radioactive Body. The Nightmare Amalgam has a radioactive aura. This functions as the sickening radiance spell that is centered on and moves with the Nightmare Amalgam, it spreads 30ft in any direction from the body. This does not require concentration and is not affected by spells such as Dispel Magic. 

Amalgamation of Minds. The Nightmare Amalgam can concentrate on up to three spells at a time. If it falls below 400 HP it can concentrate on two spells, and only one below 200 HP. If it fails a concentration on one spell, it loses concentration on all active spells. 

Relentless Nightmare (Mythic Trait; Recharges after a short or long rest). When The Nightmare Amalgam is reduced to 0 hit points, it does not die or fall unconscious. Instead it regains 200 health, then purges the head of Medomai in a necrotic burst (CON 21 save or take 8d10 necrotic damage), it then loses its spellcasting abilities. The humanoid form atop its shoulders then dissolves and a Shadow Horror appears by way of a Greater Dimir Keyrune.

Scranton Reality Anchor. The Nightmare Amalgam cannot be forced to leave whatever plane it is on unless the anchor is destroyed. Perception 25 to notice it (after it is triggered the first time) AC 30, HP 50, Immunity to all damage except force damage. 

Legendary Resistance (3/Day). If The Nightmare Amalgam fails a saving throw, it can choose to succeed instead. 

Four Legs. The Nightmare Amalgam cannot be knocked prone. 

Magic Weapons. The Nightmare Amalgam's weapon attacks are magical. 

The head of Medomai (Spellcasting). The Nightmare Amalgam is a 12th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 21, +13 to hit with spell attacks). The Nightmare Amalgam has following Cleric spells prepared:
Cantrips (at will): Sacred Flame
1st level (4 slots): Inflict Wounds
2nd level (3 slots): Spiritual Weapon, Blindness/Deafness 
3rd level (3 slots): Spirit Guardians*, Dispel Magic
4th level (3 slots): Banishment, Death Ward*
5th level (2 slots): Flame Strike, Dawn
6th level (1 slot): Harm
* The Nightmare Amalgam begins combat with these spells already active.

Multiattack. The Nightmare Amalgam makes three attacks. One with its Wounding Fist, one with its Triceratops Arm, and one with its stomp. It can replace any of these with an Exploding Lance attack. 

Wounding Fist. Melee Weapon Attack: +19 to hit (2d8 + 10) bludgeoning damage, The Nightmare Amalgam then uses a spell slot to cast Inflict Wounds as part of the fisting.

Triceratops Arm. Melee Weapon Attack: +19 to hit 28 (4d8 + 10) piercing damage.

Stomp. Melee Weapon Attack: +19 to hit 26 (3d10 + 10) bludgeoning damage. 

Exploding Lance. Ranged Weapon Attack: +9 to hit, range 300 ft., 1 target. Hit: (4d6 + 10) piercing damage + 3d12 force damage.

Quick Shot. The Nightmare Amalgam makes an Exploding Lance attack, with a reduced +4 to hit. 

Voice of Medomai (3/Day). The Nightmare Amalgam forces Medomai's head to roar. Each time it roars before finishing a long rest, the roar is louder and the effect is different, as detailed below. Each creature within 500ft of The Nightmare Amalgam and able to hear the roar must make a saving throw.

First Roar. Each creature that fails a DC 21 Wisdom saving throw is frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Second Roar. Each creature that fails a DC 21 Wisdom saving throw is deafened and frightened for 1 minute. A frightened creature is paralyzed and can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. 

Third Roar. Each creature makes a DC 21 Constitution saving throw. On a failed save, a creature takes 44 (8d10) thunder damage and is knocked prone. On a successful save, the creature takes half as much damage and isn't knocked prone. 

Disperse Heat. When The Nightmare Amalgam takes fire damage, it redirects that damage and shoots a bolt of fire at every creature within 30ft. Each target must make a DEX 21 save or takes half the triggering damage. No damage on a successful save.

The Nightmare Amalgam can take 3 legendary actions 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 Nightmare Amalgam regains any spent legendary actions at the start of its turn. 

Lance (Costs 1 Action). The Nightmare Amalgam uses its Exploding Lance attack.

Spell (Costs 1 Action). The Nightmare Amalgam casts a spell of 4th level or lower. 

Goring Rush (Costs 2 Actions). The Nightmare Amalgam moves up to its walking speed then uses its Triceratops Arm attack which does an extra 10 (2d8) piercing damage. 

If The Nightmare Amalgam's mythic trait is active it can use the options below as legendary actions for 1 hour after using Relentless Nightmare.

Lancing Barrage (Costs 1 Action). The Nightmare Amalgam makes three Exploding Lance attacks.

Terrorize (Costs 2 Actions). The Nightmare Amalgam targets one creature it can see within 120ft of itself. The target must make a DC 21 Wisdom saving throw. On a failed save, the target takes 38 (7d10) psychic damage and is frightened for 1 minute. 

Go Berserk (Costs 3 Actions). The Nightmare Amalgam moves up to its flying speed and uses its Stomp action four times.