Coalescent

Innate Spellcasting. The Coalescence innate spellcasting ability is Charisma (spell save DC 21, +14 to hit with spell attacks). It can innately cast the following spells, requiring no components and no concentration:

At will: eldritch blast (4 beams, +6 bonus to each damage roll)

1/day: dominate monster

1/day: circle of death

Forms: The Coalescent begins all encounters in the Form of Chains. At the beginning of the second round roll 1d4 to determine what form the Coalescent takes. If you roll a 4, the Coalescent assumes another form in addition to the current one. Roll 1d4 again until you get a roll of 1, 2, or 3 and add that form's effect to the current one until the beginning of the next round.

Form of Chains: The sound of whispered moaning emanates from the Coalescent as icy black chains appear from underneath its cloak wrapping it in chilling armor. AC Increased by 2, Slayer's Razor applies the Slow effect as per the spell.

Form of Fire: In a flash of darkness the Coalescent become wreathed in dark purple flames that seem to absorb more light than it gives. When weapon damage is dealt by the Coalescent any creature dealt damage is considered on fire and will take 1d6 fire damage at the beginning of each of its turns until the creature takes an action to put them out. Add 1d10 fire damage to all weapon damage rolls made by the Coalescent.

Form of Rage: AC decreased by 4, The Coalescent grows horns and loses the ability to cast spells. It makes 4 weapon attacks per multi-attack while in this state. It takes half damage from all physical attacks.

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

Magic Resistance. The Coalescent has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Coalescent's weapon attacks are magical.

Inevitable. The Coalescent's form can not be changed or altered in any way and its speed is not effected by any magic, effect, or terrain. It may pass through solid objects at will but can not stop moving until it has exited the objects space.

Subdue: While any creature is restrained by the Coalescent's chains it is silenced and has disadvantage on constitution saving throws.  

Return to Nothing. When the Coalescent is reduced to 0 hit points it implodes forming a singularity were it died, in a 5 Ft cube for 1 minute. All creatures within a 30 foot radius must make a Strength saving throw DC 21 or be pulled into the singularity, suffering 30 points of irresistible Void damage every round until freed or the effect ends. If the creature is brought to 0 hit points this way it is consumed by the singularity and dies. Any creature that failed may make another strength check at the end of their turn or as an action to escape. At the end of this effect, if the Coalescent was killed by a Runed Blade, a Shard of the Void will be left behind granting whoever consumes it great power.

Multiattack. The Coalescent attacks twice with its choice of Slayer's Razor and the Grip of Carceri.

Slayer's Razor. Melee Weapon Attack (Halberd) : +14 to hit, reach 10 ft., one target. Hit:  25 Slashing damage, and the target must succeed on a DC 21 Constitution saving throw or be subjected to the ability of the current Form.

Grip of Carceri: Ranged Weapon Attack (Whip-Chain) : +14 to hit, range 30 ft, one target. Hit:  The target is considered restrained. Whenever the Coalescent takes damage, half of the damage taken is also taken by those entangled with these rusty iron chains. There is no limit to how many chains the Coalescent may produce. Each chain has 25 HP

Chains Of Agathys(Recharge 5-6). Each creature other than the Coalescent within 120 feet must succeed on a DC 21 Dexterity saving throw or take 25 Cold damage and be petrified and restrained by spiked chains of black ice. The affected creature takes 25 Cold damage at the start of each of the Coalescent's turns. The creature can make a DC 21 constitution saving throw at the end of each of its turns. One success ends the petrified condition only. A restrained creature must make a contested Strength or Acrobatics check against the Coalescent in order to be freed of the chains. The chains can be dispelled or shattered only by a Dispel Magic or Shatter of 5th level or higher, or equivalent ability.

Feed on Weakness. When any number of creatures within 40 feet of the Coalescent fails a saving throw, the Coalescent gains 10 temporary hit points.

Cold Reckoning. When the Coalescent is dealt damage by an attack it may use its reaction to  release a burst of cold in a 10 radius sphere around itself. Creatures caught in this effect must make a DC 21 Strength saving throw or take 12 points of cold damage and be pushed 10 feet away from the Coalescent and be knocked prone. On a successful save the creature takes half damage and is not pushed or knocked prone.

The Coalescent can take 4 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Coalescent regains spent legendary actions at the start of its turn.

Move (Costs 1 Action):  The Coalescent moves in any direction it chooses up to its speed.

Cantrip (Costs 2 Actions). The Coalescent casts one cantrip. 

Multi-attack (Costs 2 Actions). The Coalescent makes a multiattack.

Absorb (Costs 3 Actions). Each creature restrained by the Coalescence takes 20 (6d8) Cold damage, and the Coalescent gains 10 temporary hit points per creature restrained.