A sword with a blade made of metal, or the least, it looks metallic, shiny and clean. A sword as old or probably older than our civilization, it has writings on the blade, never seen before, never seen in any part of the world. The sword looks alive, it has eyes of iron or what appear to be iron eyelids, one eye for each side of the crossguard. Upon holding the sword, great pain will surge through the hand and travel upward to the head, a stream of pain like a river flowing through the body, a pain so strong, more than once people have screamed and shouted like children, but when the pain is over, the sword and the swordsman fuse together or, at least, the swordsman becomes a tool of the sword.
It seems the sword will always overwrite the will of the swordsman by following their subconscious ideals and not their external intent and emotions. In the hand of a childish but loving man, the sword will never kill or injure another but only defend its user, even during emotions of vengeance, despairing hatred and frustration. In the hand of a man filled with dread and remorse, in the hand of a broken soul, it will vent and release all the hate within at once and kill anyone, even those they may love and cherish.
This sword can turn a broken hero into a despicable villain and a reluctant criminal into something holy.
When holding the sword for longer, the link between flesh and blade becomes stronger, the limbs move faster, the eyes see faster and the mind thinks faster, time will slow down allowing for more planned strikes and parries. A swordsman who never puts down the sword will hold godly powers, even be able to predict the trajectory of incoming bullets, almost read minds and see the future in simulations in their dreams, but that swordsman will become more and more a weapon and less and less alive.
After days of not holding the sword, the link is broken and the flow of pain will repeat once again when grabbing the sword, as the jealous and affectionate sword punishes you for leaving it.
The sword is a parasite created by the ancient ones, it doesn't come from this planet, the ancient ones never stepped foot on this planet, the sword hit the planet while falling in a semi-destroyed spaceship, a cargo spaceship transporting antiquities for a museum. The reason the cargo was never recovered is unknown, it has been many millennia since the fall. The sword, together with the rest of the cargo was an important piece of their history, now forever lost in this planet. Something must've happened to the old ones if they never came back for the cargo.
How could a living biological sword that attaches to any living being survive for so much, endure so much abuse?
-The sword is a bio-robot, made with genetic coding not with computer coding.
-Whether the sword is conscious and sentient depends on your definition of consciousness, to some people consciousness is an illusion created by intelligence, to others it is the result of intelligence and some even believe it comes from the soul, something separated from flesh. I'm not here to answer that question.
-The sword is a unique masterpiece done by an artisan, it has seen blood and many battles but never used en masse in any war or military force.