Sith power. He tried to stop a group of Jedi from killing him by trying to do a ritual to escape his body. Only it went wrong and he got trapped on Yavin IV. Everyone thought he was dead until he showed up again. A couple thousand years of isolation didn't help him any. His spirit worked on Luke's apprentices. It was before we married but I know enough. He drove Kyp Durron to attack Luke. Luke's spirit was separated from his body and Kun led another apprentice to destroy his body. Other apprentices stood against him to protect Luke. It took Luke's spirit and a Jedi spirit from ancient times to put him down for good.
He even tried hauling out an Imperial superweapon, the Sun Crusher,during his rampage.
Like I said, I don't think he's gone full Sith yet, the way he sounded here on the comm, though he could be hiding it...Sith are great at that as you might know. I don't trust him though.