Lamashtu was, along with her fallen sisters, a demon goddess of hell. In the words of the renowned exorsist John Constantine, she is a bit of a glorified vampire.
Back in the days before Adam and Eve were banished from the Garden of Eden, Lamashtu turned down Adam's proposal for marriage before Eve said yes and chose eternity as goddesses of hell over a lifetime of obedience on Earth just like her sisters Lillith, Durgia, and Naamah.
It is implied that her influence on Earth diminished after the Great Flood, as the catastrophic event purged plenty of hell-alligned beings and that since then she has only left hell to eat her favorite prey, newborn children, before returning back to hell.
Lamashtu started working for a group of ancient and powerful warlocks called the Brujeria and one of her orders were to kidnap babies that descended from the family line of one of their members, whose son and subsequent children left the cult and fled from Chile to Mexico. To this end, she disguised herself as a nun and infiltrated the convent where one of those babies was about to be born and kidnapped the child after killing his mother.
This attracted the attention of an occult dabbler within the convent, who summoned the help of John Constantine to rescue the baby. After succeeding to kidnap another child from the family line her identity was discovered by John but she fail to silence him when he stabbed her in the chest, forcing her to escape.
Eventually, John managed to track her down and rescue the baby but realized that the Brujeria needed the children alive so Lamashtu couldn't let them die. So, he threaten to kill the baby if she didn't tell him what were the Brujeria plotting and what was the Rising Darkness. She replied that the Brujeria found a way to break the boundary that kept Earth and Hell separate and that this would allow demonic creatures like her to roam freely across the land.
In the end, John used an amulet of Pazuzo to teleport Lamashtu away from them.