Of Denied Nature. The Abyssal Bloom can be seen perfectly while within magical darkness created by Denied Reality, regardless of how far away it is.  The Abyssal Bloom is immune to damage as long as it is not in direct sunlight.

Legendary Resistance (3/day). 3 times per day the Abyssal Bloom may choose to succeed a saving throw that it would fail.

Infected Tar. When struck with slashing damage, the Abyssal Bloom's tar attempts to protect it, granting it temporary hit points equal to half the damage dealt and gaining +2 AC for as long as those temporary hit points exist.  When the temporary hit points are removed, the tar lashes out at the attacker if they are within 30 feet of the Abyssal Bloom.  The attacker must make a DC 18 Dexterity Saving throw or take 2d4 necrotic damage and be poisoned for 1d4 hours.

Frosted Advantage. The Abyssal Bloom's attack rolls get a +1 bonus for each stack of Frostbite on the target (see Lair Actions for Frostbite status effect).

Tear Reality (Mythic Trait; Recharges after a Short or Long Rest). When the Abyssal Bloom reaches 0 hit points it does not die or fall unconscious.  It instead regains half of its hit points and it's attacks and abilities are enhanced.  Its AC is increased by 2 and no fires can be lit within 60 ft. of it.

Multiattack. The Abyssal Bloom makes three attacks with its vines.  It can replace one of these attacks with its Crush or its Bitter Frost.

Vines. Melee Weapon Attack: +11 to hit, reach 40 ft., 1 target. Hit: 18 (2d10 + 6) bludgeoning damage and moves the target 20 ft. towards it, or in any direction if the Abyssal Bloom is within its lair.  When the Abyssal Bloom's Mythic Trait is active this also does an additional 3 (1d4+1) cold damage.

Crush. Melee Weapon Attack: +11 to hit, reach 5 ft., up to 3 targets if they are all within 5 ft. of each other.  Hit: 26 (3d12+6) bludgeoning damage.  If the Abyssal Bloom's Mythic Trait is active the targets must also succeed a DC 18 Constitution save or be incapacitated during their next turn.

Bitter Frost. The Abyssal Bloom strengthens the cold in the area around it.  Each creature within 15 ft. of the Abyssal Bloom with Frostbite stacks must succeed on a DC 12 Constitution save, with the DC being increased by 2 for each stack of Frostbite.  If they fail they take 3 (1d4+1) cold damage.  When the Abyssal Bloom's Mythic Trait is active, this deals 5(2d4+1) damage on a failed save and half as much on a successful one, with the creatures also gaining a stack of Frostbite if they fail.

Abyssal Blizzard (Recharge 5-6). Each creature in a 40 ft. radius of the Abyssal Bloom must make a DC 20 Constitution save, taking 28(8d6) cold damage, gaining a stack of Frostbite, and being restrained by frozen tar on a failed save and only take half as much damage on a successful one.  Creatures who are poisoned automatically fail this saving throw and have vulnerability to the damage dealt due to the tar in their body freezing them in place.  When the Abyssal Bloom's Mythic Trait is activated, recharge this ability.  It now kills people who would fall unconscious to this ability, freezing them solid in ice.

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

Sheer Cold (Costs 1 Action). The Abyssal Bloom forces a creature wielding a metal weapon, shield, or armor to make a DC 16 Constitution Saving throw (increased by 1 for each stack of Frostbite) or take 5 (2d4+1) cold damage.  If the item was a shield or weapon they must drop the item or take this damage again at the start of their turn.

Infect (Costs 1 Action). The Abyssal Bloom deals 9 (3d4) necrotic damage to each creature within 120 ft. of it that is suffering from the poisoned condition.

Snuff Out (Costs 1 Action). The Abyssal Bloom puts out all fires within 60 ft. of it, including magical fire.  Creatures within 10 ft. of the Abyssal Bloom suffer from the effects of the Silence spell until its turn.

The Abyssal Bloom may use these as Legendary Actions when its Mythic Trait is active.

Shattering Cold (Costs 1 Actions). The Abyssal Bloom forces a creature wielding a metal weapon, shield, or armor to make a DC 16 Constitution Saving throw (increased by 2 for each stack of Frostbite) or take 5 (2d4+1) cold damage.  If the item was a weapon they must drop the item or take this damage again at the start of their turn.  If the item was a shield or armor, the next time that player is hit by bludgeoning damage the AC bonus given by the armor or shield is reduced by 2.

Frozen Tar (Costs 2 Actions). The Abyssal Bloom deals 9 (3d4) cold damage to each creature suffering from the poisoned condition within 120 ft. of it.

Suffocating Silence (Costs 2 Actions). Each creature within 20 ft. of the Abyssal Bloom suffer from the effects of the Silence spell until its turn.