After being gifted the planet Mustafar by his Master Darth Sidious, Vader was also given a huge trove of ancient sith relics and artifacts to fill out his new castle. Among the relics were Sith holocrons, amulets but also the Mask of an ancient Sith Lord called Darth Momin. Lord Momin lived millenia before the time of Vader and was a master architect and craftsman, taking care to be precise when forging his helmet.
Following this, Vader took the helmet into a dark side cave on Mustafar and peered into the past getting a glimpse at Darth Momin’s life and was so captured by what he was seeing that he took his own mask off, risking his life and placed Momin’s helmet on. The Sith spirit of Momin had hypnotized Vader and invaded his mind.
At this point, the dark side rushed through Vader and so much hatred and anger flowed through him that he immediately ripped the helmet off and managed to break Momin’s grasp on his mind, vowing to never let it consume him again…