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Grommash "Grom" Hellscream was the chieftain of the Warsong clan, and best friend and chief advisor to Warchief Thrall. He was the first orc who drank the blood of Mannoroth the Destructor, thus binding the Horde to the Burning Legion — and he would eventually pay the ultimate price to free himself and his people from the curse.]
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Upon learning the truth, Thrall and Grom set out to hunt down Mannoroth and located the vile demon in what is now Demon Fall Canyon in Ashenvale. Bellowing in rage, the Warchief struck out at Mannoroth with the Doomhammer, but the demonic leviathan shrugged off the attack and stunned Thrall with a vicious counterattack. After being taunted by Mannoroth that they were the same, Grom charged him and sank Gorehowl into Mannoroth's chest, shattering the breastplate and slicing through to the infernal heart of the demon. Mortally wounded, Mannoroth exploded in an inferno of flame, and Grom took its full fury alone. In a burst of hellfire, Mannoroth the Destructor was dead...
there is a video on warcraft3 of that fight.