Susan Scarbo was a member of the Sisterhood of the Church in Germany circa WW2. During the war she was one of the nun's who assisted the Nazi soldiers that had been killed in battle's transition into Heaven. In 1943 when HYDRA defected from the Nazi's, Susan joined them and became the Mother of HYDRA's religious beliefs, which led her to be known as Mother Night. She later on became close with the Red Skull, so when he desired an heir, she was willing break her holy vow's and slept with him.
Later on during the war the Red Skull discovered that his facilities were being brought down one by one. He then deduced that if HYDRA should lose than it could take years to regain the power it once had. So the Red Skull worked alongside Baron Heinrich Zemo, to recreate a serum that would slow down the aging process. But the results of the serum show that it would need to be administered regularly in order for the effects to remain. Red Skull and Zemo then deduced that if the serum were to be administered into pregnant female's, then the fetus would gain the enhanced longevity without the need for any refills. In response to this Susan was given frequent shot's of the Infinity formula which were intended to spread to the unborn child of the Red Skull.
In 1945, HYDRA's main facility was compromised and while the Red Skull attempted to complete his plan of global destruction, Susan went into labor and was escorted out of the facility before the SSR could find her. She was taken to a secret HYDRA bunker where she gave birth to her child. She named her Sinthea, and then spent the rest of her life training her in the arts of leadership and combat so that she could lead the new HYDRA into the 21st century.
Mother Night was trained in martial arts, making her a competent hand-to-hand combatant.