Blighted 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.

Blight Burst. The blighted treant explodes when it drops to 0 hit points. Each creature within 20 feet of it must succeed on a DC 15 Constitution saving throw or take 21 (6d6) poison damage and become infected with a disease on a failed save. Creatures immune to the poisoned condition are immune to this disease.

Blight spores invade an infected creature’s system, killing the creature in a number of days equal to 1d12 + the creature’s Constitution score, unless the disease is removed. In half that time, the creature becomes poisoned for the rest of the duration.

The disease progression can be slowed if the creature succeeds on a DC15 Constitution saving throw every twelve hours.  Failure reduces the creature's Constitution by 1. A critical success on the saving throw means that the creature's body has fought off the infection. A critical failure reduces the creature's Constitution by 2. Constitution is recovered at a rate of 1 point per long rest.

Blight Spray.  When the blighted treant is struck by a melee attack the attacking creature must make a DEX Saving throw DC. 15 or get sprayed by the poisonous blight pouring from the treant's body, taking 7 (2d6) poison damage on a failed save, or half as much on a successful one.

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.

Rock. Ranged Weapon Attack: +10 to hit, range 60/180 ft., one target. Hit: 28 (4d10 + 6) bludgeoning damage.

Animate Trees (1/Day). 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.