Curse is a hidden effect that is innately imbued on a very small number of weapons.
The Curse Auxiliary Effect allows the player to damage the Ghosts of New Londo without needing to be cursed. If using a cursed weapon two-handed or in the left hand, it can also be used to block the attacks from ghosts.
Despite not actually using the weapon, kicking while using a cursed weapon in the right hand will still affect ghosts.
The following is the list of weapons that possess the Curse Auxiliary Effect.
The following is a list of all the enemies which require Curse to damage.