Corrupted Manchineel Treant

False Appearance. While the treant remains motionless, it is indistinguishable from a normal tree.

Siege Monster. The treant deals double damage to objects and structures.

Toxic. The treant is extremely poisonous, creatures starting their turn within 10ft of the treant must make a DC18 Con save or be poisoned.

Cumulative Poison. The treant's poison worsens as you are continually exposed to it, each successive failed save causes a level of exhaustion.

Innate Spellcasting. The treant's spellcasting ability is Wisdom (spell save DC 16). The treant can innately cast the following spells, requiring only verbal components:

At will: Druidcraft, Poison Spray, Pass without Trace, Speak with Plants, Commune with Nature,

3/day each:  Entangle, Hold Person, Plant Growth, 

1/day each: Antilife Shell, 
Magic Resistance. The treant has advantage on saving throws against spells and other magical effects.

Multiattack. The treant makes two slam attacks.

Slam. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 16 (3d6 + 6) bludgeoning damage,  and the target must make a DC 18 Con saving throw, taking 14 (4d6) poison damage and becoming poisoned on a failed save, or half as much damage on a successful one.

Sapling Bomb. Ranged Area Attack:  range 60/180 ft., 10ft radius. Hit: 22 (4d10 ) Poison damage and becoming poisoned on a failed save. DC 18 Con Half.

Animate Trees. The treant magically animates one or two trees it can see within 60 feet of it. These trees have the same statistics as a treant, except they have Intelligence and Charisma scores of 1, they can't speak, and they have only the Slam action option. An animated tree acts as an ally of the treant. The tree remains animate for 1 day or until it dies; until the treant dies or is more than 120 feet from the tree; or until the treant takes a bonus action to turn it back into an inanimate tree. The tree then takes root if possible.

Poisonous Burst. Sap bursts from the treant's fresh wounds, DC 18 Con save or take 14 (4d6) poison damage and become poisoned on a failed save, or half as much damage on a successful one.