This means that the realm has been administered most recently by her daughter Queen Ishtar. This Realm is not the only Abyss Realm that apparently has two Queens on it. In this case, Ishtar did not kill or conquer Ereshkigal. By all accounts, Ereskigal is expected to eventually return, and assuming that the Realm survives the coming of the Draken, then she will be Queen again. And Ishtar will be – well – she will be what she has always been, I guess.
There is a thing on the 6th Abyss, and it is that thing that Queen Ishtar rules over. From what I have heard, Merlin and Lauren must have visited there, because as far as I can tell, the material they used to make the Spectre suits that the Ghost Squad now wear has to have come from this thing. Queen Ishtar names if the Pit of Life but I think I would find the name Pit of Death to be more fitting. It is an amorphous pool of blackness, that seems to devour any living thing that enters it, with the exception of Queen Ishtar, and certain other intelligent beings with a very specific and limited criteria. And I cannot tell you what that criteria is, because I am worn to secrecy. If I tell you, they will come and kill me. All I can say is, if you ever end up there, my advice is to stay out of that Pit.