Mara Jade was "The Emperor's Hand". If not for her encounter with Luke and Luuke, and had the Emperor or Vader survived ROTJ, she may well have become the next Sith in that line. She also had Luke's ESB saber. Which, along with Anakin using it to kill the youngones, could be part of why that lightsaber conjures up a spooky first encounter with itself for Rey. I think the poor thing has been quite shaken through the misuse it has had to endure.
Exactly! That's were my confusion lies. I've read a lot of Star Wars books, but never came across Bane and his "Rule of Two". It's almost impossible to catch up with it all and with Disney at the reigns, none of it matters. What really makes no sense is the Jedi using lightsabers in battle. Think about The Clone Wars...why would you have a sword to fight a bunch of robots when a blaster would be more efficient? A sword is a weapon best used against other swords. Sword against sword, 'mano a mano', knights and chivalry and all that... why would there be fields of lightsaber wielding Jedi and only 2 lightsaber wielding Sith? Not very sporting.