Darth Sion was a zombie who existed in the time of the old republic.
Little is known about Darth Sion's life before he turned in to a zombie, yet it is said that he had been attacked in the famous zombie attack which occured on his home planet, using a ship that was docked he decided to go to Korriban to find Brrrrrraaaainnnnssss and was yet dissapointed that everyone on Korriban was killing each other before he could do it himself.
He then went to Malachor V to look for more brains, and the only beings he encountered were Traya/Kreia/Kae/Handmaiden's mom and Darth 'Smoke weed' Nihilus, both were not acceptable for him to eat and him now having no purpose in life decided to be trained by Traya/Kreia/Kae/Handmaiden's mom.
The Big Betrayal.
Darth Sion aka zombie boy was getting very bored of life, yet he could not die after trying to commit suicide 279 times, and the only thing these suicidal attempts would give him is pain, which made him hate life even more.
Because of this fact he was going through some serious depression, when all of a sudden his best friend Darth 'Smoke Weed' Nihilus showed up and tried to cheer him up, they both got REALLY high and then they was trying to think of something to entertain themselves so for kicks they thought it would be fun to go around beating up old women, they finally found one when they realised their master DARTH BITCH TRAYA was indeed an old woman.
Darth 'Smoke Weed' Nihilus let zombie boy beat up DARTH BITCH TRAYA to make him feel better about his life, after this was done and Sion had beaten the living sh*t out of her he then went on to pursue the life of trying to be a Darth Maul wannabe and would only stay in contact with his buddy Darth 'Smoke Weed' Nihilus if he wanted to go on the Ravager and light up a joint (Sion couldn't do this in his own house as his mom would find out) thus making the Ravager a perfect place to smoke a joint considering there is a big giant space vaccum in the ship.