Abeloth is the antagonist of the nine-book Legends series Fate of the Jedi (2009–2012), introduced in Aaron Allston's Outcast (Del Rey, 2009) and named in the third volume. She is a Force entity of enormous power, styled the Mother, and the series positions her as a fourth member of the family the Mortis arc calls the Ones — alongside the Father, the Son and the Daughter.
An outside context problem
What she is for, structurally, is to be a threat that neither order has a category for. The Jedi cannot fight her as a Sith because she is not one; the Sith cannot bargain with her because she wants nothing they can offer. The Legends shelf had spent thirty years escalating Sith Lords, and Fate of the Jedi's answer was to stop escalating and change the kind of thing.
The Maw
The series ties her to the ancient Celestials, who are said to have assembled Centerpoint Station and the Maw cluster specifically to contain her. That is a large retroactive claim on existing furniture, and how well it lands depends on how much you liked those pieces of hardware being unexplained.
Era note
Fate of the Jedi is set around 43 to 44 years after the Battle of Yavin, past the end of any official era label. This site files her under the editorial Legacy of the Force subdivision, which is a publishing bracket rather than one of Lucasfilm's nine eras.
Canon status
Star Wars Legends. Abeloth does not exist in current canon; the Ones do, from The Clone Wars, but canon has never given them a fourth.
