Grom and his Warsong clan were separated from Thrall's fleet and made landfall on Kalimdor first. Eventually Grom and the Warsong were found by Thrall's forces as they were fighting humans. Reinforced by Thrall's forces and his Darkspear troll allies, Grom's forces managed to fend off the human incursion. Grom revealed to Thrall that the pass north through the Stonetalon Mountains were cordoned off by Alliance forces led by a girl named Jaina Proudmoore. Thrall thought it best to avoid conflict with the humans by having goblin zeppelins fly them over the pass. While Thrall and his forces went to secure the zeppelins, Grom could not contain his bloodlust and instigated attacks on the human encampments, despite being given explicit orders from Thrall to not harm the human forces. With no other choice, Thrall and his forces aided Grom in destroying all the human encampments. Grom was elated by the humans' slaughter and felt emboldened by the resurgence of the blood-curse. Thrall became concerned that Grom's bloodlust would prove to be a liability; and for disobeying direct orders from Thrall, Grom was given what was presumed to be an easy, if not orcishly demeaning mission to clear land and gather lumber for the new Horde's settlement.
However, the night elves were angered when Grom's orcs began to cut down the trees in Ashenvale Forest and attacked without explanation. He received help from the goblin Neeloc Greedyfingers who gave him goblin shredders for killing a furbolg. The Warsongs under Grom managed to successfully defend themselves and kill their attackers, but by doing so they caught the attention of the demigod Cenarius. Determined to avenge the deaths of the night elves who had fallen against the Warsong clan and protect the forest from what he saw as further desecration, Cenarius attempted to slaughter the orcs. Cenarius appeared to know something of the history of the orcs and was convinced they were "demon-spawned wretches" who deserved only death. In an exchange with the demigod during one skirmish Grom vehemently told him that the orcs no longer served any demons, but Cenarius didn't believe him and continued his attempts to kill them all. The orcs quickly discovered that Cenarius was invulnerable to conventional attacks. Death at Cenarius' hands seemed inevitable.
One of the troll witch doctors accompanying the Warsong clan told Grom he sensed strange energies nearby which might be able to aid them in defeating Cenarius. When Grom investigated he discovered a fountain of blood. This was the doing of the pit lord Mannoroth, who had originally enslaved the orcs after Gul'dan convinced them to drink his blood with promises of power. Mannoroth saw Cenarius as a formidable obstacle standing in the way of the Burning Legion, as he had been during the War of the Ancients. The demon poured more of his blood into the fountain on a suggestion from Tichondrius, hoping that the orcs would once again drink it and become sufficiently re-energized with demonic power to kill Cenarius. There was also another reason Mannoroth hoped they would partake of his blood...
When Grom brought his troops to the fountain, the witch doctor told him that he could sense that the fountain contained demonic energy. One of Grom's soldiers told him that if they drank from the fountain they would be going against everything they had been taught by Thrall. But to Grom these things did not matter. Desperate to preserve his clan from this new threat, Grom drank from the fountain and either persuaded or forced those under his command to do likewise. Driven again by the demonic energies, Grom attacked the demigod Cenarius' forces. Even the demigod himself was killed by the demon-enhanced orcs.
After Cenarius fell, Mannoroth appeared. By drinking the Blood of Mannoroth, the Warsong clan were once again susceptible to the vile pit lord's control. When Mannoroth told him this Grom was horrified, but that horror was one of his final independent thoughts before Mannoroth exercised control and turned Grom and the rest of the clan into his slaves, causing them to despise Thrall and the rest of the Horde and make war on them and the humans, under Jaina Proudmoore, whom Thrall had since allied with. Determined to save his old friend, Thrall confronted Grom after fighting through the corrupted Warsong clan. When Thrall reached him, Grom told him the horrible truth of the demonic curse. The orc clan chieftains had willingly taken the bloodlust offered by the Burning Legion. They were not so much cursed as doomed by the willing choices made by their power-hungry leaders. Thrall (with the help of Cairne Bloodhoof and his tauren) managed to capture Grom and bring him to Jaina, who worked with other magic-users from both her forces and Thrall's to perform a ritual that would release him from his blood curse.
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...
With nothing remaining of the pit lord but the shattered armor, Thrall made his way to where Hellscream lay in the dust. The crimson glow faded from his eyes; the blood curse which had damned his race for decades was finally over. With his best friend and Warchief at his side, the still-undefeated chieftain died at the site of his greatest triumph, both over the corruption of the Burning Legion and over the whispers of his own soul. With Mannoroth gone, the Legion's hold on the orcish race was forever shattered. Even those who had drank the blood of Mannoroth (willingly or otherwise) found that the curse had lifted, and that their souls were free from the shackles of demonic corruption. With that knowledge, Grom Hellscream left the mortal plane a redeemed spirit.