My money is Xal’atath being a next-level entity that the Void Lords send in to straighten things up when the Old Gods fail. She’s the clean up crew: if you’re a void being, she’s someone you never want to meet, because even being in her presence means you messed up.
I’m guessing that the Old Gods failed by even establishing the Black Empire to begin with, because they were supposed to burrow to the Worldsoul and corrupt it ASAP. They got cozy and reveled in their power, and Xal’atath got sent in to knock some heads, before getting locked up in a dagger.
This is a recurring theme in War Within: a lot of forces got close to corrupting Azeroth, but then failed. The crystal in Hallowfall almost got to Azeroth, but stopped short before making it Light; the first world tree almost burrowed to Azeroth and made it Life, but failed. The Old Gods almost won, but got power hungry and stopped rather than making it Void. Azeroth sits on a knife’s edge, with every one of the forces so infinitesimally close to winning it all.