Sins of the Father Ending

Rather than killing the Netherbrain and be rid of their mind flayer tadpole for good, a villainous Dark Urge instead takes control of the Netherbrain for themselves, just as the god of murder Bhaal wanted them to do in the first place. Worse still, they also corrupted everyone infected with the tadpole, including their companions, with their own homicidal impulses from being a Bhaalspawn, turning everyone connected to the Netherbrain into the Dark Urge's personal army ready to conquer Faerun in Bhaal's name.

Example of:
Downer Ending