Felo'melorn, Flamestrike in Common, is an ancient Highborne runeblade once wielded by Dath'Remar Sunstrider, even before the War of the Ancients. During the Third War, Anasterian Sunstrider wielded the sword in battle against Arthas Menethil, but Frostmourne severed the blade and he was defeated. Later, Kael'thas Sunstrider, last of the Sunstrider line, recovered it and had it reforged, "with magic, hatred and a burning need of revenge."
It was last seen when Arthas engaged Kael in a duel during the final battle between the Lich King's Scourge led by Arthas Menethil, and Kil'jaeden's agents led by Illidan Stormrage. Kael boasted that unlike human blades, which weakened when repaired, elven blades grew stronger when remade; this proved true, much to Arthas's surprise, when the runeblade was able to parry Frostmourne without failing. Though Kael ultimately fled with the sword, likely carrying it to Outland, its current fate is unknown.
In the RPG
Flamestrike is a large double-edged warblade, etched with elven runes of power. It is rumored to have once been Dath'Remar's favored weapon, and over many generations was passed down through the Sunstrider dynasty. Flamestrike grants its wielder an increased chance to hit, increased damage, and an increased chance of scoring a critical hit. It adds fire damage to every attack (more so on a critical hit) and also allows its wielder to cast the flame strike spell five times per day.
Felo'melorn was first mentioned in the Manual of Monsters as only Flamestrike, and then expanded upon in Arthas: Rise of the Lich King. It is not part of Kael'thas's loot table in either the Eye or Magisters' Terrace.