Abeloth... OK, what the hell was Abeloth? Let me explain. No, there is too much. Let me sum up. Abeloth was some sort of ancient female Force-using creature who used the Dark Side, drove people crazy, traveled in some sort of weird dark Dimension of Pain, and consumed people's life energy. Nobody knew what species Abeloth belonged to, but she was decidedly female. Her lady bits were unmistakable... and hideous. Kind of like Kreia. But shockingly, Abeloth was much, much older and uglier than Kreia.
In order to compensate for her extreme ugliness and wrinkliness, Abeloth used her powerful Force-endowed shape-changing ability. She could appear in any form she wanted, and she also could pose as any of the people whose life-essence she'd eaten, and she knew everything about them... PIN numbers, Facebook passwords, where their boyfriends and girlfriends had embarrassing tattoos, everything. So, theoretically, if Abeloth ate Calista Flockhart, she would instantly know everything about Harrison Ford's flabby naked body... and what it feels like to make out with Lucy Liu.
Abeloth didn't bother anybody during the Knights of the Old Republic era, or during the prequel films, or during the Original Trilogy, or later on, during the Timothy Zahn era, or during the New Jedi Order books... but then all of a sudden, she showed up really late in the game, during the Fate of the Jedi series, and became a huge galactic menace. For some reason. She drove a bunch of the young, weak-minded Jedi space craaaaaazy, which prompted Luke Skywalker and his annoying son Ben to search the galaxy for the cause of it all, but mostly just engage in witty banter.
As it turned out, Abeloth had eaten Luke's ex-girlfriend Callista Ming a while back. Er, not in a Natalie Portman way, but actually eaten his ex-girlfriend, digested her body, crapped it out, and also somehow leeched out her soul in the process. So Luke was searching the galaxy for Abeloth, but meanwhile, Abeloth knew everything about him that his ex-girlfriend did, including that embarrassing little lump on his left ass-cheek that he said was a mole, but was way too big to be a mole. In fact, Abeloth actually shape-changed into Callista Ming and tried to seduce Luke at one point. "Hey, look at me, I'm your ex and I'm still hot." And Luke was like, "No way! I'm married... no, wait, check that. Actually, my wife is dead. Dammit. But I still don't trust you." And Abeloth was like, "No, seriously, it's me, not some hideous ancient Force monster here to devour your life essence, pinky promise!" But Luke still hesitated, and finally Abeloth just gave up and jumped in his face.
And then Luke and Abeloth started fighting, and... well, the book series hasn't ended yet, so I don't really know what happens, but judging by how Legacy of the Force ended, it'll probably be really stupid, whatever it is.