In 2013, veteran video-game concept artist Paul Richards decided to put together an art book based on a fictitious IP of his own creation. He invited several of his colleagues and friends to contribute to the book. He also made an open call-for-art to the video-game art-world for contributions. The only things required were skill and imagination. There was no set lore or backstory to the IP... simply the premise of a story set in a dark-fantasy world.
Below is the process for an image I contributed to "Substrata"... The Behemoth of Wittenberg.