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

Entropic Aura. Within 300 feet of the Harbinger, probabilities break down, becoming warped beyond all recognition. In order for any creature to hit another creature, creatures must roll a number less than or equal to that creature’s AC. A roll of 20 on the die is now treated as a roll of 1, and vice versa. In order for a creature to succeed on a saving throw or ability check, it must roll a number less than or equal to the roll’s DC. (Treat the Harbinger’s proficiency bonus as a proficiency minus.)

In addition, the Harbinger’s very presence causes wild fluctuations in the Weave, causing all magic to behave unpredictably. All creatures other than the Harbinger must roll a d20 whenever they cast a spell of 1st level or higher. On a 1, that creature must roll on the Wild Magic Surge table to create a magical effect. (A Wild Magic Sorcerer may roll one additional time and pick whichever result they prefer.)

Extradimensional Walker. The Harbinger is able to traverse dimensions that mortal minds cannot begin to perceive. Functionally, it is always under the effects of the blink spell, though this ability cannot be dispelled by spells such as dispel magic or blocked by an antimagic field. 

Necrosis Incarnate. The Harbinger is an avatar of death, weakened by effects that would restore others to life. If it is targeted by a spell that would restore hit points, it instead takes radiant damage equal to double the number of hit points the spell restores. 

Multiattack. The Harbinger uses its Scream, if available. It then makes two chaos bolt attacks, three talon attacks, or two talon attacks and one chaos bolt attack. 

Chaos Bolt. Ranged Spell Attack: -6 to hit, range 120 ft., one target. Hit: 44 (2d8+10d6) damage, as if by a 9th-level chaos bolt spell. (Determine the damage type and possible new targets accordingly.) 

Talon. Melee Weapon Attack: -6 to hit, range 5 ft., one target. Hit: 28 (5d10) slashing damage. The target must succeed on a DC 13 Constitution saving throw or take 10 (2d10) necrotic damage at the start of each of its turns due to an entropic wound. Each time the Harbinger hits the wounded target with this attack, the damage dealt by the wound increases by 10 (2d10). Any creature can take an action to stanch the wound with a successful DC 13 Wisdom (Medicine) check. The wound also closes if the target receives magical healing. 

Scream (Recharge 5–6). The Harbinger unleashes a terrible screech that threatens to shatter the minds of all who hear it. Each non-aberration creature within 120 feet that can hear the Harbinger must succeed on a DC 13 Intelligence saving throw or take 55 (10d10) psychic damage and be subject to an effect from the short-term madness table for 1 minute. If that creature fails by 5 or more, it must also roll on the long-term madness table, being subject to the effect for the next 1d10 × 10 hours. A creature affected by a short-term madness effect can repeat its saving throw at the end of each of its turns, ending the effect on a success. A creature that succeeds on its initial saving throw takes half damage and suffers no other effects. Any creature that succeeds on a saving throw against this ability automatically succeeds on all further saving throws against it for the next 24 hours. A creature that cannot hear is immune to this effect. 

Time Warp (3/Day). The Harbinger causes time to fluctuate in a 30 foot radius around it. All creatures within range must succeed on a DC 13 Constitution saving throw or roll a d6. On a 1-5, the creature gains the effects of the slow spell for the next minute. An affected creature may repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. On a 6, the creature instead gains the effects of the haste spell until the end of its next turn. If a creature fails this saving throw by 5 or more, its age also changes by 1d4 × 10 years, growing that many years younger if it rolled a 1-5 on the d6, or that many years older if it rolled a 6. This effect cannot age a creature below 5 years old or over within 30 days of death. 

Plane Shift (1/Day). The Harbinger flees through a rip in spacetime to another plane as if by the plane shift. 

Teleport. The Harbinger teleports up to 30 feet away to an unoccupied space it can see. 

Unravel. As a reaction to a creature within 60 feet of it casting a spell that targets it, the Harbinger attempts to absorb that spell’s energy to strengthen itself. That creature makes a DC 13 check with its spellcasting ability. On a failed check, the spell has no effect, and the Harbinger instead regains a number of hit points corresponding to the level of the spell cast: 1d10 for a cantrip (level 0) + 1d10 for each additional level of the spell above 0. On a successful check, the spell takes effect as though nothing happened. The Harbinger cannot use this ability with healing spells. 

The Harbinger 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. The Harbinger regains spent legendary actions at the start of its turn. 

Talon Attack. The Harbinger makes one talon attack. (This action is unavailable if the Harbinger is not on the same plane of existence as the target.) 

Dread Whispers (Costs 2 Actions). One creature of the Harbinger’s choice that it can see within 60 feet of it must succeed on a DC 15 Intelligence saving throw or take 21 (6d6) psychic damage as discordant whispers in unknowable tongues assail its mind, wracking it with terrible pain. If it fails by 5 or more, it is also stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. A creature can also spend its action to break the stunned creature out of its stupor. 

Time Warp (Costs 3 Actions). The Harbinger uses its Time Warp ability, if available.