(extrakun - sourced with permission originally from here)

Hidden upon a mountain, deep in the recess of a cave, is the fabled Three-Swords Forge. The adventurers wouldn’t have discovered its location if not for a series of thefts of old swords which are hoarded by historians, scholars or noblemen. The disappearance of those swords lead the adventurers to the location of the Three-Swords Forge after a series of adventures. It turns out that those three swords are also a Warlock sword - a Vampire Sword, a Severblade and a Volcanic Sword

1. A Warlock, who happens to be a master blacksmith, is attempting to forge the ultimate sword of all - by melting down the metal and collecting the essences of the three Warlock swords, he hopes to forge one that is capable of destroying demons, forever, instead of banishing them for a mere time away from the moral plane. His motives are up to the GM and even though his foes are ultimately demons, it doesn’t make him a ‘good guy’ necessary!
2. Long ago three Warlocks adventuring together chanced upon a cavern which was an ancient library. Intending to return there to unearth its secrets, they seal the cavern with an unique lock - a magical barrier which can only be destroyed with their three swords. At the Three-Swords Forges, they forge the Severblade, the Vampire Sword and the Volcanic sword. They then depart; however, each schemes to retrieve the swords for themselves. The man or men behind those swords could be those warlocks themselves, resurrected accidentally by magic or their descendants.
3. Legends speak of a vile demon by the name of Barghaest, a three-headed humanoid monster with the faces of an old man, a troll and a hag. A warlock used each of the three swords to defeat the demon and sealed its essence within each of the blades. Those who have stolen the sword either seek to free the demon, or use it in a similiar manner.