Orcus

This being is a demon prince, not typically thought of as a deity, but it may be as powerful as a greater god. It has few worshipers among humans or demi-humans, mostly among chaotic evil humanoids and other such creatures.

When discussed, it is typically referred to by nicknames so as to not draw its attention. Common names include the Prince of Undeath, Master of Vampires, The Shadow That Was, and the Big O.