Evermillion was sealed inside Takamura's body/soul. She is arguably the most powerful witch, an endless source of mana, a deity, _______.
Tower Witches want to dominate / own her, workshop witches want to keep her locked.
Exactly WHAT or WHO is Evermillion?
I basically just trashed my other answer, but you can always see it again in the edit history: as of now she is still an unknown assailant.
Evermillion is a huge mystery. Her origins, how she came to be sealed in Honoka, why she was sealed, and even what she actually is; are completely unknown (she referred to herself as a demon with Honoka, though it's possible she was being metaphorical). Sometime in the past she was sealed inside of Honoka Takamiya with a five layer barrier preventing her from escaping. She seems to have some history with Medusa.
Evermillion (エヴァーミリオン?), fully named White Princess Evermillion (白姫エヴァーミリオン Shirohime Evamirion?), is an entity of unknown origin sealed inside Honoka Takamiya. She is addressed as the White Princess by various characters. A total of five seals were placed on her.
Source: http://witch-craft-works.wikia.com/wiki/Evermillion
Oh yeah, and theres scheduled to be an OVA as of 2015 per Wikipedia too.
