It is - as far as anyone can tell, when they first look at it - just a sword. Abnormally beautiful - with its sleek silver blade that looks thin enough to slip between a person's ribs, and an equally silver hilt with etchings that texture its surface and tell stories of purity and immortality, studded sparingly with multi-faceted rubies that shine redder than blood - but that is par for the course when something is made by the Goblins.
Somehow, when they look at the strength of the blade and the sharpness of the edges, they fail to realise that this sword, with Godric's name inscribed into its length, is dangerous; they forget that Goblins make beautiful things, yes, but they also make deadly things, and while the sword fits into both the former and latter categories, it was made to be put into the category with other dangerous, deadly, Goblin-made things.
They know that the Sword of Gryffindor, as with all Goblin-crafted artefacts, has spectacular qualities that make it superior to other weapons. Over time, they learn that - when lost - it returns to its wielder's side.
The sword has another quality that they fail to notice.
It is created by Goblins with Goblin-metal. The sword can take in that which makes it stronger. They are aware of this, but none of them - even the wise one - really realise what that means.
The sword is unaware of its ability at first, and so when Godric carries it into battle and holds it against the throats of their enemies, or swings it against their necks, or lunges forward to slide it into their chests, it comes as a surprise.
The sword kills its enemies, and it steals their souls. It draws them out through their skin and devours them. The souls linger inside it, and at first it is unbearable; they are miserable things, souls. They are lonely and loud with it. They are displeased and vocal with it. They are bloodbodysoul-thirsty and vicious in their hunger.
Over time, the sword becomes hungry, too. It begins to guide the hand of its wielder, begins to consume souls the second it touches the skin of their foes. The sword collects souls for itself, and for the souls within it, who are trapped within the cage made from its blade.
Soon enough, the sword is not the only one searching for more souls. The souls constrained within it begin to clamour loudly for the admittance of more, and with a shared goal, the sword does its best to humor them.
Godric dies, eventually, and so the sword spends centuries being summoned from one hand to the next, and it never loses its desire for the death and carnage that stealing souls brings.
It is buried in darkness for an indeterminate amount of time before it drops onto the unsuspecting head of a child, who grasps the hilt of it between two hands and thrusts its blade up.
The sword buries itself in the mouth of a basilisk that it used to know, and it leeches the snake's soul from its impenetrable body. Once the basilisk's soul finds its way inside its silver-walled cage, it curls up tight, hissing wildly to itself in fear.
Neither the sword nor its souls have any idea what has the snake in such a panic until the venom slips through the walls of the cage and spreads poisonously toward the inhabitants, reaching for them. The venom snakes into all the souls the sword has collected over the years, and it poisons them all, until they are screaming, writhing masses. And then, every last one of the souls - even the soul of the basilisk - is gone, and all that is left is the sharp lines and deadly curves of the sword, laced with a deadly toxin.
The sword loses its taste for souls, after that. It is venom-laced, and while there is strength in numbers, there is also might in solitude, and the sword - well. It is stronger with the venom lingering in the biting lines of its blade than it ever was with the souls that lived within it, desperate for more.
The venom, unlike the souls, does not look for itself within the faces of others, and - if pressed - will kill without discrimination or protest, for it hates everyone in the world equally.
The corrosive blade of the sword slices down in an arc to cleave a ring and then a locket in two, and it sweeps forward to separate the head of a snake from its body, and it destroys all four of the souls it sucks in.
Silence reigns on in the cage lined with metal and poison.