One of the many new features of Patch 10.1.5 is the addition of customization options for Warlock Pets, including two entirely new Felhunter models. The first version of this new Felhunter model, the Voidmaw customization, has been found! Thanks to Thdlock for the tip.
The Ritual of the Voidmaw Felhunter can be found in the Legion version of Karazhan, just after killing the Mana Devourer boss. Once you kill the boss and grow back to your normal size, go to the second floor of the library (you will be on the first floor after killing the boss) and on the south portion of the room, you will find the ritual page stuck between books.
Read more on wowhead.com