If Void Maw and another creature would be put into the graveyard from the battlefield at the same time, Void Maw is put into your graveyard and the other creature is exiled.

Cards exiled with Void Maw that would go to the graveyard but have that replaced by an effect such as Leyline of the Void are considered new objects. They can not be used to activate Void Maw's ability again.