Corrupted Treant

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

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

Petrifying Gaze. When a creature that can see the treant's eyes starts its turn within 30 ft. of the treant, the treant can force it to make a DC 17 Constitution saving throw if the treant isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified into wood. Otherwise, a creature that fails the save begins to turn to wood and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic. While petrified in this way, the creature is vulnerable to fire damage. Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the treant until the start of its next turn, when it can avert its eyes again. If the creature looks at the treant in the meantime, it must immediately make the save. 

Multiattack. The treant makes two slam attacks.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 16 (3d6 + 6) bludgeoning damage and 14 (4d6) necrotic damage. The treant regains hit points equal to the necrotic damage dealt.

Hurl Mud. The treant digs its roots deep into the earth and hurls a large glob of mud at an area within 20 feet of it. Creatures in a 20-foot cube must make a DC 17 Dexterity saving throw or take 4d6 bludgeoning damage and be blinded until they take an action to wipe away the mud.

Reanimate Trees (1/Day). As a bonus action, the treant magically animates one or two trees it can see within 60 feet of it. These trees have the same statistics as a corrupted 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.

Protect the Trunk. When the treant is hit by a melee weapon attack, it can use its reaction to make one Slam attack against the attacker. If the attack hits, in addition to dealing damage, the attacker must make a DC 17 Strength saving throw or be thrown 20 ft. and land prone.

Fire Bad! When the treant takes fire damage, it can use its reaction to excrete a sticky sap-like substance, coating itself until the end of its next turn. While coated in this way, the treant turns its fire vulnerability into fire resistance.