From Lexicanum

A Bonesword is a form of Tyranid close-combat bio-weaponry.

In 4th Edition Boneswords are psychic weapons which crackle with power coursing through them. Its hilt sends out a psychic pulse to nearby Tyranid units when it is stimulated (usually by combat) sending them into a killing frenzy, often succeeding in attacking an enemy even if they should by all rights be dead.^1-p31

In 2nd Edition, the Bonesword was a living creature functioning as a sword, and used in pairs; the blade itself was actually the creature's massively elongated horn, serrated on both edges. The blade is capable of gradually growing in length, as well as repairing itself given enough time. The Bonesword creature's small brain is situated deep within the weapon's hilt. The Bonesword is incapable of independent thought but can generate a powerful surge of psychic energy when stimulated by the user. The energy flows along the nerve tendrils embedded within the blade, creating a field effect and effectively making the sword the biological version of the force weapon.^2-p52

In 1st Edition, Sword-like in appearance, the actual blade of the weapon is the horn of the Bonesword creature, its main body forming the hilt area. The blade slowly grows in size as the Bonesword ages, and is capable of regenerating damaged sections of itself. A complex series of nerve tendrils wind throughout the blade of the Bonesword, and when the creature is stimulated it floods this nerve network with psychic energy, creating an effect somewhat similar to that of a charged force weapon.^3-p31

Boneswords were once the primary close-combat weapon of the Tyranid Warrior, who wielded them in pairs, but seem to have been replaced with more generic stabbing and slashing claws in recent revisions of the genus.

Sources

• ¹:Codex: Tyranids (4th Edition) (identical entry in Warhammer 40,000: Wargear p40)
• ²:Codex: Tyranids (2nd Edition)
• ³:Warhammer 40,000: Battle Manual