When Evanora was given a task to procreate, they were expecting that her descendant would bear magical powers as all witches did once they were born. But that is not what happened.
After the war, the witches along with the other supernaturals became extinct. The moment the witches were gone, it was as if the magic in the air were gone as well.
Evanora did procreate but she noticed that the children she gave birth to don't bear any magical powers. They were pure humans even though the blood of a witch was running through their veins.
At first, she thought that they were just dormant witches. So, she continued to procreate while observing the offspring she left if they would manifest any magical powers. But they didn't.
When her magical powers started to awaken one by one, she touched the souls of her descendants using the ritual that her mother taught her and realized that she was giving birth to half-witch half-humans who don't have any magical powers.
