I found this interesting.
Be warned, if you read the comics, this is probably a major spoiler.
Eventually Nihilus became so absorbed by the Dark side of the Force that his physical body began erode. Knowing he would die if he did not act soon, Nihilus ripped his spirit from his body and encased it in the armor he wore. Allowing the dark side to consume him, as his body dissolved completely. Using the force he was able to keep his robes, armor and mask held together giving him some form, allowing him to use his force powers and wield a lightsaber, but apart from that he had no physical form.
Chapter 12 of A Soul Adrift
: On the planet Felucia, a young apprentice of the Dark Side thinks back to the beginning of her training as she lies in wait for her prey...
All the Time
: After four years in the Unknown Regions, the Exile returns to the known galaxy to visit an old enemy.
: A master of the Dark Side finds himself about to lose the one thing he cares about--and he will do anything to stop her from endangering herself.