In season 2 (episode 5) of Game of Thrones we see that Red Priestess Melisandre gives birth to the Shadow child of Stannis Baratheon to assassinate Renly Baratheon. But why didn't they try to have another Shadow child? Or why didn't they use the only one to assassinate Cersei Lannister?
Stannis: Make me another son.
Melisandre: I cannot.
Melisandre: You don't have the strength. It would kill you.
Stannis: I'm not so easily killed. Men have been trying for years. I want you.
Melisandre: Your fires burn low, my king. There is another way. A better way.
Stannis: You told me your magic requires a king's blood.
Stannis: I'm the one true king.
Melisandre: You are. But there are others with your blood in their veins. You will sit on the Iron Throne, but first there must be sacrifices. The Lord of Light demands it.