The following contains spoilers for Adam Christopher's Star Wars: Shadow of the Sith, on sale now.
In the Star Wars Universe, one of the most divisive points came with Luke Skywalker's disillusionment with the Force. As The Last Jedi confirmed, he rebuked the Jedi, considering them a similar form of mental manipulation and control like the Sith. And as he educated Rey on the light and dark and the balance, fans saw how Luke distanced himself from the cosmic energy on Ahch-To.
It seemed he was struggling due to all these contradictory philosophies within. And that's not even mentioning Ben Solo becoming corrupted, killing students and destroying Luke's temple. Now, come Adam Christopher's Shadow of the Sith novel, it becomes clear what exactly cut Luke off from the Force.
In this story, Luke and Lando tracked Ochi of Bestoon to a Sith cult, the Acolytes of the Beyond. Using the aid of a defector, Komat, they found a mysterious source that haunted Luke, torturing him with visions of darkness. It turned out to be Kiza, a Sith warrior from Chuck Wendig's Aftermath trilogy of novels, who wore an ancient mask, getting possessed by a Sith viceroy known as Exim Panshard. It'd help him secure clues to find Exegol and a way back to life, which led to the heroes battling the entity on Taw Provode.
Thankfully, Luke destroyed the mask, but Kiza died in the process. However, Luke brought the mask to his temple, continuing his collection of light and dark relics with Lor San Tekka. And as he observed the fragments following the shattering, he got locked onto the dead kyber crystals. Shockingly, Luke felt the darkness calling out to him, hinting that the villain's presence might have lived in the shards.
Retaining this amongst Luke's marquee relics wasn't advisable because he kept seeing the viceroy killing folks in the past. And what made it worse was the Jedi confirmed he felt an emptiness when he tried to search for the Force. It stunned him, as he had never felt anything like that before, but it suggests this mask poisoned the master and helped sever his connection, despite his Jedi pedigree. And interestingly enough, Luke's situation got interrupted by a teen Ben, foreshadowing how he'd turn and become Kylo Ren.
The mask corrupted Kiza beyond hope and salvation, so there's a good chance this gift of power and the temptations of Exegol's secrets might have warped Ben's mind without the kid even knowing it. What's for sure, though, is that Luke made a mistake. He gambled on this being a mere disturbance that'd pass when ultimately, the mask began distracting him. It left him more vulnerable than ever and maybe even led to him ignoring signs that Ben was slipping.