Abyssus, Dragon of Entropy

Draconic Heritage. Abyssus is loyal to Varaea, and Varaea alone. All attempts at Charisma (Intimidation, Persuasion, Deception) checks against Abyssus are made at Disadvantage. 

Legendary Resistance

Herald of Entropy. 

• All Magic cast within 30 feet of Abyssus forces a roll on the Wild Magic table. 
• Structures within 1 mile of Abyssus often spontaneously rearrange and even dissolve. Each round, roll a d4. On a 1, structures spontaneously dissolve into a fine powder of their construction material. On a 2 or 3, they rearrange in odd ways. On a 4, nothing happens. 
  • Creatures inside buildings which dissolve must take fall damage as usual, if they are high enough to do so. 
• This feature includes spells cast with Abyssus's Innate Spellcasting.

Chaos Incarnate. All creatures who start their turn within 60 feet of Abyssus must make a DC25 Wisdom saving throw. On a fail, they must use their action to attack the nearest creature (other than Abyssus) they can see on each of their turns for 1 minute; on a natural 1, they attack themselves with Advantage. Creatures who fail this save can re-roll at the start of each of their turns, and creatures who succeed on the initial save are immune to this effect for 24 hours. 

Shifting Planes. Within 30 feet of Abyssus, all damage is converted to one of the following: Force, Radiant, Psychic, Necrotic, Fire, Cold, Lightning or Thunder. This damage type is determined by a d8 roll at the start of Abyssus's turn. Additionally, Abyssus rolls a d20. On a 1, he is Vulnerable to this damage type for the round; On a 20, he is Immune. All other rolls cause no change. 

Innate Spellcasting. Abyssus's casting ability is Dexterity (Spell save DC 25, +15 to hit). He can cast the following spells without requiring Vocal, Material, or Somatic components: 

At-Will: Dancing Lights, Plane Shift, True Polymorph, Minor Illusion 

17/day each: Guards and Wards, Teleport, Imprisonment 

7/day each: Alarm, Time Stop, Astral Projection

9/day each: Storm of Vengeance, re-roll on d100 table for spells 

• Spells which are a base of 6th level or higher require two rolls on the Wild Magic table, taking the worse outcome for Abyssus. 

Note: These spells were the result of 1d20 for per-day usages, and 1d100 per spell (4/3/3/2). If this suits your table, feel free to use this rolled list or roll your own. If not, then the author suggests giving Abyssus a total of 10 casting slots per day. Roll 1d100 for the spell (use a randomly generated list, or craft your own), and 1d10 (ignoring 10s) for the spell level. 

Chaotic Resistance. If Abyssus fails a saving throw, he can choose to succeed instead. The creature who forced the saving throw rolls 1d48 and on a 1 suffers the effects instead.On an 8, Abyssus fails the save. Other rolls result in Abyssus succeeding on the save. 

Multiattack. Abyssus takes a Field of Entropy action or a Cone of Chaos action. He then takes 2 Claw and 1 Bite attack. 

Bite. Melee Weapon Attack, +15 to hit, Reach 15ft, one target 

Hit: 3d12 damage (Shifting Planes type)

On a critical hit, the target is automatically Swallowed (ignoring recharge). 

Claw. Melee Weapon Attack, +15 to hit, Reach 20ft, One target 

Hit: 3d8 damage (Shifting Planes type)

Swallow (Recharge 4-6). On a successful Bite attack, Abyssus swallows the target. The target must succeed a DC25 Strength saving throw or be Grappled, Restrained, and Blinded, taking 6d8 damage (of the type determined by Shifting Planes). After being Swallowed, roll 1d8. On a 1, the creature is banished to one of the planes of the Multiverse, returning in 1d4 rounds at 1 HP, in a location of their choice within 30 feet of Abyssus, within sight. On an 8, the creature is teleported out of the dragon to a point within 30 feet of his sightline, regaining all HP from the Swallow damage, and 1d12 temporary hit points. On a roll in between, the creature is Swallowed as usual. This may only apply to one creature per Swallow per round. They can repeat the Strength saving throw at the end of each of their turns, and take 4d8 additional Shifting Planes damage at the start of each turn while Swallowed. On a success, they are ejected 20 feet from Abyssus's mouth, taking 2d8 Shifting Planes damage and Prone. 

Field of Entropy (Recharge 6). Abyssus forces a roll on the Wild Magic table. All creatures within 30 feet are affected by this roll. Creatures who do not have the Spellcasting ability roll twice, and take the effect which has a worse outcome for themselves. 

Cone of Chaos (Recharge 4-6). Breath Weapon, 120ft cone 

Abyssus unleashes a mighty cone of pure chaotic energy. All creatures in the cone must succeed a DC25 Dexterity saving throw or take 12d12 damage of the same type as the current Shifting Planes damage. 

Take Flight. Abyssus moves up to his Fly Speed without provoking Attacks of Opportunity. 

Entropic Gaze. Abyssus casts Counterspell at 6th level. If the spell is countered, the target rolls on the Wild Magic table twice, taking the worse result for themselves. 

Abyssus can take 3 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. abyssus regains spent legendary actions at the start of her turn.

Chaotic Wave. Abyssus forces a creature he can see to make a Concentration check on a spell which requires Concentration. 

Dissipate. Abyssus focuses on a creature it can see. The target must succeed a DC25 Constitution saving throw or take 10d10 Force damage as their very essence explodes into a field of probability and chaos. On a success, the creature takes no damage, is Immune to this effect for 24 hours, and gains a +2 bonus to their AC for 1 minute. 

Erratic Teleport. Abyssus teleports to a random location in a 1km square. (DM note: Roll 1d1000 x 1d1000 for new position.) If he lands in the space of a new creature, he is shunted to the nearest available location. 

Entropic Annihilation. When reduced to 1/4 HP, Abyssus collapses into a probabilistic nightmare. All rolls within 1 mile are made at disadvantage, including his own, as probability and physics disintegrate. This effect may only be countered by Honoris's Word of Law ability, by speaking the phrase "All is correct here."