Hear me me out…: 
Alleria accepted the void.
In the coming expansions we see Xal making everything to push her further into despair (to save her friends, family etc) and every time she fails she “asks the void” for a little more power.
Of course this is all Xal’s plan to make Alleria give herself to the void entirely)
At some point the final confrontation between Alleria and Turalyon happens; seeing Alleria completely gone he still isn’t capable of killing her and losing her forever, so Turalyon turns to some benevolent void entity (maybe Locus walker) and learns about…The blade of the black empire.
He acquires the blade and as a last effort to save Alleria, he locks her there in hopes to purify her one day…
This being the moment Alleria de facto becomes Xal’atath.
The blade then is lost or stolen and sent as a void weapon to the past (black empire) where she remains a servant (hence Xal asking N’zoth to finally free her to fulfil her purpose/destiny)
The blade then ages and arrives at where we are today in time. And Xal is free.
But she has a problem…she knows Alleria (which is herself) will get imprisoned by Turalyon.
Her goal here is keep pushing herself (Alleria) to become more powerful, but also making sure her “both selves” will be there this time to stop Turalyon from imprisoning them.
This explains why Xal seems to know so much about Alleria’s future, even showing her the confrontations she will have with Turalyon, and why she mentions “having had another mortal body” in the past.
I think that is Xal showing Alleria their own memories. They’re not illusions… they’re memories.
And when Xal successfully defeat Turalyon and save herself (Alleria) from imprisonment, Xal abandons her current body, merges with Alleria and they become again Xal’atath - but unimaginably more powerful.
Oooooof, I hope this isn’t too confusing, And thanks anyone that reads until here xD, remember this is speculation for the fun if it
What you guys think