Where darkness hides (black butler)

She is the creature of destruction, discord, and strife. She is from darkness, she lives in darkness, she is darkness. Ciel and Sebastian come across a young girl while out investigating a murder case, finding her in a blood bath she had caused. Instead of handing her off to the queen or killing her, they take her back to the Phanthomhive estate. Why? Because she is a half-ling. A demon half-ling.