After suffering a number of defeats at the hands of the Ñoldor in Beleriand, Morgoth, Glaurung's creator, came to realize that his armies of Orcs were no match for the Noldor unaided. As such, he began to experiment with ways to create new, deadlier creatures for use in his armies. Glaurung was the result of these experiments. After a century of brooding and growing in the dark pits of Angband, Glaurung first emerged in the year FA 260 and attacked the Elves maintaining the Siege of Angband. Though Glaurung was not of the winged race that would later arise, he was the greatest terror of his time. He burned and ravaged Ard-galen, the land of the Elves in Hithlum and Dorthonion. However, he was not yet at his full strength, so was driven back by Fingon, the prince of Hithlum at that time, and his archers. Morgoth was displeased with Glaurung for revealing himself before he had grown to full strength, as Morgoth had planned for Glaurung's existence to be a complete surprise.
To Glaurung, this attack was merely a youthful testing of his strength. Terrible as he was to the Elves, the armor of his scales was still vulnerable to the bite of weapons. However, despite his untimely foray, Morgoth's plans were not disrupted.
Glaurung was contained in Angband for another 200 years before he was again loosed during the Fourth Battle of the Battles of Beleriand. Known as the Dagor Bragollach, so-called for the fires unleashed by Morgoth from the Iron Mountains, Glaurung in full power led Melkor's forces against the High Elves of Beleriand. Now fully grown, none could withstand his might, and with the Balrogs, the demon servants of his master at his side, Glaurung led the Orcs to an overwhelming victory and broke the siege of Angband.
In the fifth battle, the Nirnaeth Arnoediad, Glaurung caused even more destruction, as he had fathered through unknown means a brood of lesser Dragons to follow him into battle. So a great army of Elves and Men fell before the onslaught, for none could withstand the Dragonfire save the Dwarves of Belegost, who had come to battle a common foe.
After the Elven and Edain armies had been broken, Morgoth used Glaurung to hold the territories he gained; but force in battle was not the only power the monster knew. He brought many under his sway with the binding power of his serpent eye and hypnotic Dragon Spell, powers that served him well as one of Morgoth's greatest champions.
After Glaurung aided Morgoth during the Battle of Unnumbered Tears, he was given the assignment to complete the Sack of Nargothrond, one of the Elven fortresses in Beleriand. Glaurung came upon Nargothrond with great force, and sacked the kingdom easily. He faced Túrin, son of Húrin, at the gate and so he froze Túrin with his eyes. Túrin heard the screams of Finduilas, daughter of Orodreth, as he stood there. When the captives had been taken away, Glaurung unfroze Túrin and gave him two choices: the first to follow Finduilas, and the second to aid his mother and sister, who were supposedly suffering in Dor-lómin. Túrin chose the latter, and in this way he was deceived by the dragon, for his mother and sister were living well in Doriath. Glaurung then gathered up all the treasures of Felagund and hoarded them deep within Nargothrond's underground halls, he then sat atop his hoard, guarding it jealously, and so he rested. Finduilas's fate was sealed by this as she was later killed by Glaurung's army.
Glaurung then received knowledge of Morwen and Nienor's departure to seek their family members. In the forest where they were travelling, Glaurung found Nienor and caused her to lose her mind and memories. This caused her to run through the forest "like a deer". Soon after this, Túrin found her crying on Finduilas's grave. Not knowing that she was his sister, Túrin named her Níniel (Tear maiden) and took her to his home with the folk of Brandir. There they lived for the next three years, during which they fell in love, a terrible curse brought about by the dragon's sinister machinations.
After those years, Glaurung attacked the area around their home. Túrin resolved to kill the dragon. Two men went with him. One became afraid and fled, and the other was crushed by rocks. However, Túrin was able to kill Glaurung by thrusting his sword, Gurthang, into Glaurung's belly, his one vulnerable spot. Glaurung felt his death wound and screamed. When the dragon's toxic blood touched Túrin, he fell into a swoon. Glaurung screamed in pain until his strength was gone. Nienor found him there, with Túrin beside him. With his last breath, Glaurung gave Nienor her memory back, and died. Shortly thereafter, Nienor jumped into the river below, and Túrin awoke and threw himself on his sword, succumbing to the dragon's curse.