Keilyn's Lost Lullaby

Keilyn opened her eyes one day and she just ... was. Not knowing she was destined for things greater than all of those around her, she wandered the world alone in child form until she was taken to the House of Deveraux.
Something was very familiar about the Elder Devil, but she didn't know what it was until she had her first dream.
Lucy fell shortly after.
How did a child go so wrong and become something that all of the Above and the Below would come to fear?