Tharzog is the chieftan of Mount Gram. Mount Gram was the previous stronghold of the Rohirrim, or Eothed, before they imigrated to Rohan. After they left it un-guarded, the orcs and goblins of the mountains probably claimed it for their own. But when goblins gather, they usually end up squabbling and fighting with each other, unless some bold and strong goblin steps up as a chief or captain. Tharzog is one of those few bold enough to claim leadership.
Tharzog himself held Mount Gram, and populated it with many orcs and goblins. The strongest and mightiest orcs and goblins would be forged into his own personnal guard, and held a fiercesome reputation. When Agandaur was sent North to muster the orcs of the Mountains, Agandaur sent a personnal invitation to Tharzog and his army at Mount Gram. When the invitation reached Tharzog, he couldn't resist. Tharzog, along with any orc, goblin or uruk, wanted to go to war then stand around an old hill fort. So, he mustered his strength, and marched off to Fornost (were most goblins were gathering) with great hast. In their rush, they were recklass, and the Sons of Elrond (Elladan and Elrohir) soon picked up their trail and stalked them all the way to Fornost.
When they arrived, Agandaur made Tharzog and his elite guard his own personnal guard, and they stood guard in the citadel for many days... untill the Fellowship of the North showed up. The Fellowship (Eradan, Andriel and Farin) fought their way through the entire city, slaying all who stood before them. But the goblins who stood in their way were the weak-links of the force. The Fellowship had trouble, though, when they entered the citadel. Agandaur was just dispacting Tharzog and his guard to deal with whatever was going on outside. The Fellowship, along with the Sons of Elrond, were silently creeping into the upper tower when Tharzog noticed them. He closed the gate leading to the upper champers, which seperated the Fellowship and the Sons of Elrond. The Fellowship chose to stay and fight Tharzog while the Sons followed Agandaur. Tharzog then shouted for his guards, blew his orc-horn, and jumped down into the arena below to fight the Fellowship, which was not a very wise decision.
The Fellowship was eager to fight a worthy oponent. Farin put it, "...I'm tired dealing with lackeys...". But the would find Tharzog is more than a worthy oponent. His might and strength proved too powerful for the Fellowship. They couldn't fight him face-to-face, as his sword blows proved un-blockable. And to make it worse, his elite guard showed up for the fight, too. Since the couldn't block his blows, the Fellowship dodged them, swiftly striking him with powerfull blows then rolling away to not get hit by his strikes. Soon enough, they slayed him, striking him down in the citadel chamber at Fornost. Tharzog and his elite guard were powerful, but couldn't hope to stand up against the Fellowship of the North.