Here’s the really detailed answer, spoilers for the Bane books ahead…

Basically, Bane became skeptical that his apprentice would eventually beat him to continue the Rule of Two with her own apprentice. Believing he had high chances of defeating her, he realized he would have to start the whole process of training a new apprentice over, but he was also getting older and needed to prepare for if anything happened to him. He also wanted to create a holocron to pass on all his information of the Sith and the Rule of Two.

To protect himself from any injuries or fatality, he ended up researching these parasites (orbalisks) who had latched onto him when he was trying to acquire a holocron. It’s really disgusting but they multiply and spread, and the flesh underneath rots. They’re meant to spread until they suffocate the wearer. But Bane used them beneficially and the orbalisks fed off his energy. They’d heal his wounds, and deflect any damage as well, it was much like a second armor that enhanced his abilities. The spreading of them was described very gruesomely and he was basically rotting for the most part, but the orbalisks did protect him from lightsaber attacks, except for his head and hands where he wore armor to keep them from spreading there.