The Black Citadel

There's a scroll that gives some meaningful knowledge about the dark ones:

The dark ones are a group of dark magi, serving the dark lord Wiculoth. Wiculoth handpicks his most trusted servants himself, and those that are entrusted with the honour of 'dark one' bestowed upon them, receive the gifts of the dark way, making them extremely powerful in the dark arts of magic. Though one must give their very soul to Wiculoth in order to gain blessed dark immortality.