Bloodtainted Greatsword

Legend says that after slaying the venomous dragon,
The famed hero bathed in its blood, for he desired invulnerability.
Sure enough, he became untouchable.
Be it swords, be it spears, or be it rain of arrows,
He laughed and shrugged them off.
Eventually, his foes learned:
He had borne his prized greatsword on his back as he bathed
And now, his back bore a great weak spot, the very shape and size of that sword.