This is about unlocking boss and some sub boss weapons.

Unlockable	How to Unlock
Butcher's Cleavers	Defeat Butcher in Eclipse Mode.
Hermit's Swords	Defeat Hermit in Eclipse Mode.
Lona's Katana	Defeat Lona.
Lynx's Claws	Defeat Lynx in Eclipse Mode.
Shogun's Daisho	Defeat Shogun in Eclipse Mode.
Trickster's Nunchachos	Defeat Trickster.
Wasp's Naginata	Defeat Wasp in Eclipse Mode.
Widow's Fans	Defeat Widow in Eclipse Mode.