God grew in Heather Mason's womb, due to Claudia Wolf's plotting and Valtiel's constant vigil. Claudia also torments Heather throughout the game, letting God feed off of her anger and despair, believing that a deity who is familiar with such emotions would be kinder and more empathic than one born of happiness. The birth of God ensures the death of the host mother.
At the end of the game, when Heather finally confronts Claudia, she takes a capsule of Aglaophotis, which then forces God out of her body. After Heather throws up the writhing, puny fetus, Claudia, in a fit of desperation, devours it. She is wracked with pain and a similar reddish "rash" that Heather had encountered engulfs her body, and is then pulled into a hole by Valtiel below, where Heather follows.
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Heather vomiting and killing God.
Once Heather reaches the bottom of the hole, she finds that she is in the same room as God, which has grown to gigantic size in an amazingly short period. Valtiel appears once more to shroud God's head before vanishing. Heather sees Claudia's empty robe on the floor and remarks "You can't be dead! I was gonna kill you!".
After a fight, Heather finally kills the abomination. She kicks God multiple times in the face to ensure She is dead, then drops to the floor and cries, releasing her emotions throughout her journey, lamenting Harry.