A battlesword is the most powerful type of sword. It is also the heaviest (8 lbs) and most costly sword.
The sword can cut limbs apart if the hits are solid (a good 50% chance with solid and very hard hits) This causes the NPC to lose blood at an alarming rate and fall unconscious. Happy "hunting" !
Like all swords, it must be traded, and it's rare and expensive. The most costly sword.
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes the battlesword thusly:
Any large sword intented for two-handed use. Most battleswords have broad, straight blades 48-60" in length.