I am not sure what is going down with this Artos guy in SM Stirling's Emberverse series. His Sword was not made in this world. When he wields it in battle, he loses himself in divine and cosmic visions and kills with every blow. Seeing his face, the enemy scream, flee or kill themselves. He prophesies that most who flee will not survive. The Sword never needs to be cleaned or sharpened. Quarrelling allies who look at each other through the crystal of its pommel see each other's sincerity and make peace.
That is quite a powerful Sword. As with any other magical or supernatural instrument, we need to know what it can and cannot so so that the narrative does not give us a deus ex machina further down the line.