Green Celestial Treant (Corrupted)

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. 

Corrupting Aura: Usable once per short rest. As an action, you can release a wave of negative energy as a 15-foot burst centered on yourself. All hostile creatures within the area take 1d8 points of necrotic damage and cannot be the target of a healing spell until the beginning of your next turn. This damage increases by 1d8 for every 5 character levels you possess, for a maximum of 5d8 at 20 level.