Frostmourne was a rune blade that the Ner'zhul orc, the Lich King, had torn from the Frozen Throne. According to Ner'zhul, Prince Arthas Menethil - his unwitting potential owner - was to discover a sword on the Northrend continent. The Frostmourne was in the hands of the prince until the very end, until her blade was split into four parts after the battle in the Icecrown Citadel.
When Kil'Jeden, one of the three eredar leaders of the Burning Legion, imprisoned an old orc in the Throne of Ice, he chained his disembodied spirit to magic armor and a rune blade, forever imprisoning him on top of a huge rock, the so-called Ice Citadel. Both the armor and the blade were made by natrezimami, the servants of the eredar, in order to contain and control the Lich King.
Ner'Zul's Plan
But the old shaman was not at all satisfied with his new position and began to develop a plan for liberation. The first stage of his plan was activated when he used his powers to push Frostmourne across the ice from the citadel and plunge it into one of Northrend's ancient caves. After which it was necessary to wait for the one who would take the sword. The lord of the dead saw his savior in Arthas Menethil, the young prince of Lordaeron, who arrived in these lands to put an end to the plague epidemic and his personal enemy - the wounded Malganis.
Enslavement
Muradin Bronzebeard, the head of the dwarf research expedition, who was in Northrend at that time, found a mysterious artifact that gives great power. When the army of the Terror Lord Mal'Ganis began to outnumber the forces of the expedition, Arthas and Muradin decided to use this force.
In the depths of a hidden ancient cave, the Lich King’s sword was guarded by a ghost guardian who tried to warn the young paladin about the dangers of the damned sword, but the prince, furious, ignored all warnings and killed the keeper. When the sword was found, Muradin realized that the blade was cursed, but Arthas announced that he would gladly accept any curse in order to save his homeland. After these words, the ice cover of the sword split, and the blade burst out. A jagged splinter hit Bronzebeard and, as many thought then, killed him. The prince, not paying attention to the loss of a friend, threw off his hammer, Revenge of the Light, and grabbed the runic blade. From now on, his soul belonged to the King of the dead.
The fall of Lordaeron
With a sword in his hands, Arthas returned to his camp and launched a counterattack against Mal'Ganis, quickly destroying the fortifications of the dreadlord. Natresim ran into the prince, telling him that the King of the Dead knew that he would find the damned sword, and that now he had stolen the soul of a young paladin. He spoke as if he believed that the prince was now on his side, but Arthas used the rune blade to kill the lord of horror at the command of his new master.
Now, becoming the death knight of the Scourge (army of the dead), Arthas killed all the soldiers and officers in his camp and resurrected them with the power of the accursed sword of the Lich King. Thus, he created his own invasion army before returning to his homeland, where he was greeted with a royal welcome. Upon arrival, the fallen prince headed straight to the throne room, where he killed his own father, King Terenas, and let the hordes of the dead into the streets of the city. Quel'Thalas, Dalaran, and several other kingdoms of the continent fell behind Lordaeron.
Coronation
Some time after the Battle of Mount Hyjal, Illidan the Demon Hunter used a powerful spell to destroy the Throne of Ice. Although his plan failed, the spell still widened the crack through which Frostmourne was pierced, and the power of the King of the Dead began to fade, so he recalled his faithful knight to Northrend. There Arthas defeated the demon hunter and ascended the spire to the Ice Throne. Then he used the sword of the Lich King to break the ice, and put on the crown of Nerzul, thus merging with his master.
Overthrow of the king
During the last battle of heroes with undead troops, the cursed blade was smashed by the Ashbringer, which was owned by the paladin Tirion Fordring. Thousands of souls locked in a blade fell upon the defeated King when he was finished off by heroes who united to put an end to his reign. A few moments before his death, the fallen prince saw the ghost of his father.
"This ... is still ... not the end ..."
But the story of Frostmourne does not end there. The former elite of the army of the dead, the death knights, went to the site of the fall of the King of the dead to find the wreckage of the sword. The shards guarded four powerful ghosts. They all fell at the hands of the death knights. The fragments were reforged at the top of the Ice Citadel into a new artifact - the blades of the fallen prince. Who knows what awaits the new owner of the blades? Perhaps very soon we will learn the continuation of this story in the games of the Warcraft universe, and the Lich King’s sword will again begin to enslave the souls of mortals.