Grommash Hellscream is the Father of Garrosh Hellscream and quite worthy of his own tale. This is what Thrall says about him:
"In many ways, the curse of our people began and ended with Grom. His name meant "Giant's Heart" in our ancient tongue. He earned that name a hundredfold as he stood alone before the demon Mannoroth — and won our freedom with his blood. Lok'Tar Ogar, Big Brother. May the Warsong never fade."
Grom’s story starts on Draenor where he was Chieftain of the Warsong clan. Looking at Grom and his son there are many similarities; he was brave but brash, running into danger headlong without much planning and quick to underestimate his opponents. But Grom was a great warrior, and even though his mistakes lead to the death of many of his clan, and eventually the enslavement of all orcs, he always seemed to make it through, although not unscathed.
The biggest part of Grom’s tale deals with the Burning Legion and the Blood of Mannoroth. The plan was to get the orcs to drink it with the promise of power and victory over their enemies but there was a catch they didn’t realize until much later. Much like the Blood Elves’ addiction to magic, the orcs would become addicted to the Fel, and controlled through it by the Legion.
It did give them power though, it made them stronger than ever and also grew their young at an unnatural rate, aging them almost before their very eyes. The bloodlust is what they called it when the urge to kill came upon them. It made them little better than mindless beasts but some thought the trade for the immense power was worth it. And the chieftains spoke for their people. When one drank, they all drank. And Grom was the first to drink. To accept the power and all that it entailed. He doomed his people to slavery. To the Burning Legion. But they didn’t know that. Not yet.
Gul’dan knew. He knew everything the Horde was sacrificing, everything they would lose. He preyed on the chieftain’s pride, on their lust for power, on their distrust of the Draenei for his own designs.
When the spirits and the elementals abandoned them that should have been their first clue that they were heading down a dark path, but the lies of the warlock and the lure of power blinded them, some believing they had no choice but to join as their clan had, and others seeing the land die around them knowing they were too far now to turn back.
When Grom finally understood, when his people were weak and listless around him, sapped of the will to fight when the Black Portal came down and they were cut off from the Fel, he alone persevered through sheer strength of will. He met Thrall, became his advisor, and worked on freeing the orcs from their captors and rebuilding the horde. But as smart and able as Thrall was, ultimately he was at best half-orc. He had been raised by humans and seemed to think more like them than his own kind. He and Grom continually butted heads and finally Grom went against his Chief's orders entirely, finding a new source of fel and making the same choice he’d made before. Make the orcs powerful once again, defeat their enemies, rule this new world and make everyone witness their supremacy.
But this time Mannoroth revealed the truth, that drinking his blood bound them to him, that their power was, in fact, his own.
"Stupid, pitiful creature. I am the rage in your heart. I am the fury of your thoughts. I alone empowered you to bring chaos to this world, and by the endless void, you shall!"
Only by magic, and with the help of the Alliance, including Jaina and her mages as well as Cairne Bloodhoof, was Grom freed from the bloodlust. Realizing what he had done, and how he had doomed his people, he made the ultimate sacrifice. With his great axe Gorehowl he attacked the gigantic Mannoroth, and slew him. But in the battle he was mortally wounded, and died with the knowledge that he had freed the orcs. With the death of Mannoroth the curse was broken, the Legion’s hold on them shattered. He died in Thrall’s arms.
I think Grom was almost a father figure to Thrall, an orphan raised by humans he was among the first orcs Thrall had ever met outside of captivity. He represented a life that Thrall had never known, and didn’t understand. When he met Grommash’s son, Garrosh, I think he felt a responsibility to him. He certainly told Garrosh all about his father's sacrifice and then took him back to Azeroth, keeping him by his side. The two orcs represented a very large blind spot to Thrall. They both caused a huge amount of trouble and pain to him but they were brave, and by their own logic they were doing what was best for their people.
For a comparison Grommash Hellscream is rated about the level of Uther Lightbringer in importance of Lore and to their respective peoples.
He’s also part of the reason orcs and Night elves will never get along as he was the one who killed Cenarius the Demi-god