Castor's many diviners, sorcerers, and witches have uncovered proof that the Calixis sector is home to an extraordinarily powerful Chaos entity:  Balphomael, the Horned Darkness

Whether you want to call it an unbelievably powerful daemon or a minor god is debatable. In terms of influence in the local area (Calixis in particular, and to a lesser extent neighbouring sectors and territories) he is definitely god-like. It is believed that this daemon have been part of Calixian mythology and religious life for as long as there have been people out here. Around 20.000 years in other worlds. As far as your librarium and templum can determine the entity is not so keen on breaking through into reality, but rather more focused on creation cults and gaining worshippers.

The Broken Angel has a small, abandoned shrine that may have been dedicated to Balphomael once. There is also a very brief mention of a horned darkness  in a fragment of Lorgar's writings, so the Primarch may have encountered it or its minions during his great pilgrimage. The writings of the proto-deacons are rather more interesting: They mention the Horned Darkness as a power to be placated, but not worshipped. Various holy texts go on to explain that the Horned Darkness is an insidious and jealous thing, prone to subverting the faith of those that interact with it (which isn't good as he deacons Lorgar inspired were to worship chaos undivided, not a single daemon-deity).

Note: Balphomael is not the same entity as the wicked Crow Father!