Green Celestial Treant (Midwood)

Charge. If the treant moves at least 20 feet straight toward a target and then hits it with a maul attack on the same turn, the target takes an extra 1d6 bludgeoning damage. If the target is a creature, it must succeed on a Strength saving throw against your spell save DC or be knocked. 

Primal Bond. You can add your proficiency bonus to any ability check or saving throw that the treant makes. (Included)

Maul. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 1d8 + 6 Bludgeoning damage. 

Poisoned Fungi. Usable once per short rest. As a reaction, when you are struck by a weapon attack you can release a dose of poisonous spores upon your attacker. The attacking creature takes 1d6 points of poison damage, the attacking creature can make a Constitution saving throw against your spell save DC to take half damage. This damage increases by 1d6 every other character level for a maximum of 10d6 at 20 level.