Frostmourne was the runeblade that Ner'zhul, the Lich King, thrust from the Frozen Throne, intending that Arthas Menethil would discover it on the continent of Northrend. It was wielded by Arthas until it was destroyed by Tirion Fordring. No one knew where the remnants of Frostmourne and its battered hilt were for some time.
In World of Warcraft: Legion, a frost death knight can gather the shards of Frostmourne and reforge them into two powerful new runeblades - Icebringer and Frostreaper, the Blades of the Fallen Prince.
Frostmourne has the ability to drain life from its victims — and to drain the soul of anyone whom it connects to. The Lich King is capable of seeing through the eyes and hearing through the ears of anyone who wields Frostmourne; this ability cannot be stopped by any mortal means. The Lich King can communicate with the wielder at will telepathically, on any plane and at any distance. The Lich King uses this ability to try and corrupt the wielder over time. This has several effects. An individual who wields Frostmourne will not part with it willingly. Over time the person will go from good to neutral and finally to evil. A non-undead evil wielder will then become undead. Finally the sword is able to suck the being's soul into the sword. Frostmourne then retains the memories and skills of its victim. In fact, as long as the wielder's original body remains within one mile of Frostmourne, the sword maintains control of the body, and he may not be completely aware that his consciousness has shifted to the weapon. Beyond one mile, the sword cannot control the wielder's body, and the wielder of course finally becomes quite aware of his current state (if he is not already).
The Lich King can take possession of a soulless body that comes in contact with him and Frostmourne at the same time. The soul within Frostmourne is destroyed unless it willingly merges with the Lich King, becoming lost as a separate entity for all eternity. If a soul merges with the Lich King, he gains all the skills and memories of the victim. He has only done this one time, with Prince Arthas Menethil.
The following notable characters were slain by Frostmourne or otherwise lost their souls to it. With the possible exceptions of those brought back as undead creatures, it is presumed that their souls remained trapped in the sword until its destruction.
Arthas Menethil (Warcraft III; first soul claimed)
Captain Falric (killed in Northrend, Arthas: Rise of the Lich King; resurrected as a death knight)
Captain Marwyn (killed in Northrend, Arthas: Rise of the Lich King; resurrected as a death knight)
King Terenas Menethil II (killed in the Imperial chamber of Capital City, Warcraft III)
Gavinrad the Dire (killed near Andorhal, Warcraft III)
Sage Truthbearer (killed near Andorhal, Warcraft III)
Ballador the Bright (killed near Andorhal, Warcraft III)
Uther the Lightbringer (killed near Andorhal, Warcraft III)
Sylvanas Windrunner (killed in Quel'Thalas, Warcraft III; resurrected as a banshee)
King Anasterian Sunstrider (killed on the Isle of Quel'Danas, Arthas: Rise of the Lich King)
Dar'Khan Drathir (killed on the Isle of Quel'Danas, Arthas: Rise of the Lich King; resurrected later)
Genjuros (killed in Alterac Mountains, Warcraft III)
Haomarush (killed in Alterac Mountains, Warcraft III)
Khanzo (killed in Alterac Mountains, Warcraft III)
Mazrigos (killed in Alterac Mountains, Warcraft III)
Throk'Feroth (killed in Alterac Mountains, Warcraft III)
Jubei'Thos (killed in Alterac Mountains, Warcraft III)
Landazar (killed in Dalaran, Warcraft III)
Conjurus Rex (killed in Dalaran, Warcraft III)
Antonidas (killed in Dalaran, Warcraft III)
Halahk the Lifebringer (killed in Lordaeron palace gardens, Warcraft III)
Magroth the Defender (killed in Lordaeron palace gardens, Warcraft III)
Jennalla Deemspring (killed in Lordaeron palace gardens, Warcraft III)
Sapphiron (killed in Northrend, Warcraft III; resurrected as a frost wyrm)
Baelgun Flamebeard (killed in Azjol-Nerub, Warcraft III)
Blood-Queen Lana'thel (killed in Northrend, Wrath of the Lich King; resurrected as blood-queen of the San'layn)
Overlord Drakuru (killed atop Voltarus, Wrath of the Lich King)
Dranosh Saurfang (killed in Northrend, Wrath of the Lich King; resurrected as a death knight)
According to King Terenas Menethil II's spirit, there were thousands of souls contained within the blade. Countless souls were freed from the broken blade, but others were not so fortunate.
Theoretical stats for Frostmourne as a World of Warcraft item were shown during the Lore Panel at BlizzCon 2007, likely as a joke. The quote on the item tooltip, "Hail to the King, Baby!", is a phrase popularized by Bruce Campbell's Ash at the end of Army of Darkness.
The developers said they were "still working the fine details" and that the Equip effect was "shot down by various powers-that-be" (illustrating the potential problem with a screenshot of the new Lich King, Private Ownusohard). However, they closed with "we'll get something to you guys, we promise."
During a Class Panel Discussion at BlizzCon 2009, Blizzard developers revealed that the "something" in Icecrown Citadel was a new legendary axe, [Shadowmourne].