Botulinum Snakeroot, the Sentinel Sextet of Chaorum

Sentinel Sextet (con). This creature has advantage against spells and other magical effects. It also has immunity to any effects and damages from a constitution saving throw.

Dead on Heal. When this creature is dealt necrotic damage, this creature instead heal equal to the damage rolled this way.

Mythic Resistance (5/day). If Botulinum Snakeroot fails a saving throw, it can choose to succeed instead. 

Stench. Any creature that starts their turn within 30 ft from this creature must make a constitution saving throw with disadvantage against DC 27. If they fail, they gain the poisoned condition, and gain Toxic II (see description). If they succeed, they instead take 5d4 poison damage.

Tree puppeteer. When you would take damage, choose any tree within 60ft of you. That tree take half of that damage rounded down, up to their maximum hit points.

Undead Fortitude. If damage reduces Botulinum Snakeroot to 0 hit points, it must make a Constitution saving throw with DC of 5+ the damage taken, unless the damage is radiant or from a critical hit. On a success, this creature HP becomes 20 hit point instead.

Spellcasting. Botulinum Snakeroot is a 23rd-level spellcaster. Its spellcasting ability is constitution (spell save DC 27, +19 to hit with spell attacks). Botulinum Snakeroot has following Sang Sain spells prepared:
At Will: Tasha's Caustic Brew, Primal Savagery, Tasha's Mind Whip, toll the dead, Counterspell
4 times per day: armor of agathys acid (precasted), hunter's mark, silvery barbs

3 times per day: Acidic Bolt, dissonant trip, Bless, bane.

2 times per day: Raulothim's Psychic Lance, Acid Ball, disintegrate,

1 time per day: Cloudkill, wish, druid grove (pre casted)

Multiattack. This creature uses 5 attack of either Nature Sword and Acid Sting. It also can swap one of them to cast a spell without any components.

Nature Sword. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 9 (1d6 + 5) piercing damage + 19 (2d12 + 5) necrotic damage. The target gains Bleeding I (see description) and Necrosis I (see description).

Acid Sting. Ranged Weapon Attack: +19 to hit, range 60/200 ft., 1 target. Hit: 16 (2d4 + 10) Acid damage. The target gains Sour I (see description) and Toxic I (see description).

Absorb. Choose one creature within 120 ft of Botulinum Snakeroot with Toxic type token(s). That creature removes all Toxic type token(s) that they have and roll d4s equal to the tokens removed this way. Deal poison damage equal to the result, then Botulinum Snakeroot recovers hit points equal to the damage taken this way.

Acid Protection. if this creature was hit with an attack within 100 ft, if the attacker has Sour type token(s), you may remove 1 or more of that type tokens and roll d4s equal to the tokens removed this way. Reduce it to the total roll, potentially becoming a miss on the attack.

This creature can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.

Tree Stride (Costs 1 Action). This creature moves equal to her movement speed. This movement does not provoke opportunity attacks.

Acid Sting (Costs 1-3 Actions). This creature uses one Acid sting action. Hit or miss, the target and each creature within 5 ft of it must make a DC 27 dexterity saving throw or take 2d10 piercing damage per Legendary action(s) you use for this attack.

Reactive (Costs 2 Actions). This creature regains its reaction.

Spherical Defense (Costs 3 Actions). this creature may spend its hit dice (up to its constitution modifier) and add them as a temp HP. While the defense is up, this creature cannot be targeted by spells of 5th level or lower. The temp hp works only for Snakeroot and is not resistance or immune to any damage. It also cannot be transferred via Tree Puppeteer feat.

Liquidify. As this creature falls to the ground, a slime like creature forms. This is Botulinum Snakeroot's real form. Amorphous and full of toxin. Snakeroot recovers HP equal to any leftover hit dice it has. For every hit dice rolled, add your constitution modifier.

Your Reactive Legendary Actions now costs only 1 Action.

You are a Huge Ooze and your skin toughness are weird and only can be attacked by attack rolls that ranging from 14-17. Critical hit still hit, and critical misses still misses. If a creature hits you from the inside, your AC becomes 14.

You have resistances against all damages.

It loses Tree Puppeteer and Stench feature.

It gains new features:

Splashing Acid. Any creature misses their attack towards Botulinum Snakeroot gets splashed with 2d4 acid damage if it's within 10 ft of this creature.

Rusty Air. Any creature that starts their turn within 30 ft of Botulinum Snakeroot gains Sour I and Toxic I. Any creature instead gain Sour II and Toxic II if they start within 10 ft of this creature.

It looses Nature Sword action.

It gains new actions:

Swift Strike. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 21 (4d6 + 5) Bludgeoning damage + 12 (1d12 + 5) necrotic damage. The target is now restrained. This creature cannot restrain more than 2 creatures at a time.

Energy Squeeze. Melee special attack: Creatures restrained by Botulinum Snakeroot must make a DC 27 constitution saving throw or take 21 (3d12) necrotic damage and reduce your constitution score by 1d4 (adjust your maximum hit points accordingly). Creatures take halve damage when succeed and take no constitution reduction. This creature then heals equal to the damage dealt by the necrotic damage. 

This creature has 5 Legendary actions to use now.

Amorphous stride (Costs 2 Actions). you can try to occupy another creature's space. That creature must make a DC 27 strength saving throw or be restrained inside Botulinum Snakeroot. You can restrain up to 4 medium creatures or 2 large creatures inside Botulinum Snakeroot. This mythic action does not count against your Swift Strike restraining limit.

Bounce (Costs 2 Actions). This creature jumps without provoking opportunity attacks to an empty space within 30 ft. Then each creature within 10 ft of it makes a DC 27 dexterity saving throw or take 4d4 acid damage. Creatures take halve damage when succeed. Then, each creature within 10 ft of this creature must choose one of these effects:

• Their Armor Class reduced by 1 (up to 5).
• Their weapon attack rolls reduced by 2 (up to 10).
• Their speed reduced by 5 ft (up to 15 ft).

This effect last until you repair your armor or weapon using Mending, or take any form of healing for the speed reduction. Each casting of mending removes 1 stack of armor class reduction or 2 stack of attack rolls reduction. Each level of healing removes 5 ft reduction of speed. Each rarity also recovers your speed reduction by 5, starting from common.