Dragon's Bane

Dragon's Bane is an enchanted halberd +3, which inflicts 1d10+3 points of piercing damage, and as its name implies, it deals an additional 6 points of damage when it's used against dragons. It has +3 thaco bonus, it weighs 12 lbs and the weapon has a speed factor of 9. This item appears in Baldur's Gate II: Shadows of Amn. It's not known where this weapon can be gained.

Halberd +3 has a speed factor of 6, which means that is weapon is only better than the generic enchanted halberd when its used against dragons. For all other enemies, the enchanted halberd +3 would be better, because it has a higer rate of attacks.

In-game description
"Halberd +3: Dragon's Bane This was the weapon of Thorvin Dukal, one of seven self-proclaimed Dragonslayers who spent years railing against what they called "an insidious dragon infestation that threatened Faerun." Thorvin had this weapon enchanted at great expense and used it on many occasions, though of dragons only a single white has fallen to it. The Dragonslayers met their match in a cantankerous ancient red who slew the entire group in an epic battle. This weapon did not end up in the beast's hoard however, as the creature returned to its lair to recuperate and when it emerged the field of combat was well scavenged."