Felo'Melorn is an an ancient runeblade that once belonged to Dath'Remar Sunstrider himself. Passed down through generations, it was eventually broken by Arthas when he dueled Anasterian Sunstrider. However, Kael'thas Sunstrider later found the sword, and reforged it, making it even stronger.
In game, the sword is one of the few "Artifacts". It grants the wielder +20 Damage, and use of a firebolt spell. It causes a fiery glow on the hero, letting your enemies know which hero is holding the sword. Keep the wielder alive!