Abeloth's story takes place in a series of Star Wars Extended Universe novels called Fate of the Jedi. It's a nine-book series that stars with Luke and Ben retracing the steps of Jacen Solo to try and figure out what caused him to embrace the Dark side. As they follow his steps across the galaxy, the Lost Tribe of Sith shows up, the Imperial Remnant keeps trying to gain what power it can, and Abeloth starts to exert her influence. At the beginning of the series, the writers keep hinting at this oppressive, evil atmosphere invading the galaxy. By the end it's pretty obvious: Abeloth.