In several Forgotten Realmsnovels, Godsbane was a magical sword which was actually a manifestation of the deity Mask and was wielded by the then-mortal Cyric during the Time of Troubles. Eventually, Cyric learned that the sword he was holding was indeed the god of thievery in disguise and broke it, dealing a grave blow to the god's power.
Cyric named the sword"Godsbane" because while he was a mortal, he used it to slay Bhaal (then Lord of Murder), and after becoming a deity, he used it to kill Leira.
Godsbane needs to consume the energy of souls to survive.
Before she came into the grasp of Cyric, Godsbane had driven all of her previous wielders mad, forcing them to sustain her by killing victims.