The Misfit Mage and His Darling Demon by MN Bennet

Wally and Bez have spent the last six months eluding the Collective together. Not that the mages have spent much time pursuing the dangerous devil and his misfit mage, since even the best magics can’t match a Diabolic’s strength.
When Bez surprises Wally with an anniversary trip to a special event hosted by the Fae, Wally is presented with a chance to work on rare artifacts belonging to a Fae noble. However, Bez learns the Fae may be connected to the recent abductions of fellow demons, allowing them to crossover into various Hell dimensions—and threatening to expose his secret.
Bez is not the unstoppable devil that everyone believes he is. If the Collective learns about Bez’s deceit, they will rain down the full force of their magical resources to kill him. Wally and Bez will need to work together to unravel the mystery behind the missing demons and study intricate dimensional magics all while traveling in a magical villa through space and time.