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.