Vile Mothers are hideous creatures living in the Pits of Hell. They only appear in Act IV. Due to their ability to spawn offspring, many players prefer to go after these monsters right away, especially when there are multiple Mothers in the area; players risk getting swarmed by massive numbers of foes if they try and deal with anyone else. Vile Mothers also have a nasty slash attack they will use when in melee range. Vile Mothers can be revived by skilled Necromancers, though of course they will no longer spawn Vile Children.
No one knows from where these sickening beasts came, but it is universally wished that they would return to that undoubtedly foul pit. As if dealing massive damage with their razor sharp claws were not enough, they also spawn smaller - but no less violent - offspring to rend the flesh from the bones of their victims.