Bal'dan, the Horn of Shivergut

Aggressive. As a bonus action, the tanarukk can move up to its speed toward a hostile creature that it can see.

Magic Resistance. The tanarukk has advantage on saving throws against spells and other magical effects.

Fiendish Blows. The Tanarukk does an additional die of damage when it hits with an attack (included in attacks)

Grappler. You have advantage on attack rolls against a creature you are grappling. You can use your action to try to pin a creature grappled by you. To do so, make another grapple check. If you succeed, you and the creature are both restrained until the grapple ends

Great Weapon Master. On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.
Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.

Daunting Build. Bal'dan has advantage on all athletics checks '

The Horn. When Bal'dan is knocked to zero hit points, he raises back up with 75, his size becomes Huge, he adds an additional weapon die to all damage, he becomes resistant to bludgeoning, piercing, and slashing damage, and his AC increases to 22.

Fatal Charge. When Bal'dan is knocked to zero hit points once again in the form of The Horn, he drops to 1 hit point, his size become gargantuan, he gains yet another additional damage die to all damage, his AC increases to 24, His movement is tripled, he can dash normally as a bonus action, he becomes immune to any mental effects & physic damage, and if he has his reaction, he may be able to take two immediate greataxe attacks.

Multiattack. The tanarukk makes two attacks: one with its bite and one with its greataxe.  Then an unarmed attack as a bonus action

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 17 (2d8+8) piercing damage.

Greataxe of Returning. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 21 (2d12+8) slashing damage.  Can be throw 20/60, and if thrown it returns back to its wielder's hand.

Unarmed Strike.  Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 15 (2d6+8) bludgeoning damage.  Upon hitting a creature, Bal'dan can decide to grapple a target, DC 20 Strength saving throw required to escape the grapple.

Unbridled Fury. In response to being hit by a melee attack, the tanarukk can make one melee attack with advantage against the attacker.