Dragon's Tooth is one of the best bastard swords in the game (1 handed and 2 handed), with a great speed and reach, rivaling even the feared rune weapons.
To obtain this weapon you need to successfully defeat and capture the king of Ravenstern (King Gregory IV). Once this is done, engage on a conversation with him. If his kingdom has 6 or less castles+cities left, he will have a special option in which he trades his unique weapon for his freedom. This will remove the weapon from his inventory meaning the next time you'll meet him, he will use his secondary weapon.
This can only be done if the player is a Monarch.