Absolum - spoiler free secret end boss

 Thanks to this Steam forum person:

   Spoiler-free path to hidden boss

If you want to unlock one of the games hidden rituals and fight one of the two hidden bosses in Absolum, you'll have to make your way to the island of the dead and through the haunted manor.

At the end you will find a room of passage with mysterious symbols and constellations above.

Upon thoroughly exploring you should find a second pathway hidden behind the main exit to the right. Follow it to the end and the hidden path should reveal itself.

Good luck.