In case you missed it (it triggers during a random hand-in during Hallowfall campaign):
Alleria is asked "Xal'atath is from the Black Empire?? One of the Old Gods?"
And she replies: "No.. Something else. A survivor from the depths of time."
So I think we can rule out the 5th Old God thing for her.
Honestly the most surface level thing for her probably IS a Void-aligned Dreadlord, given her tactics and behavior and armor (I mean Archmage Drenden was Nathrezim 101). They've been known to possess corpses like her too. Her quotes about them are usually either disappointed (varimathras) or almost impressed (mephistroth).
I was thinking for a while she was a full-on dark Naaru or being similar to that, given her silhouette and Beledar connections- but really what's to stop a Nathrezim from doing the same naaru-absorbtion that Alleria did? So, even if it feels cringe or odd, Xal's 'brethren' may not be Old Gods after all.
Shes keen on harvesting our Fury, our 'Wrath'. Revendreth has a missing Harvester of Wrath who was supposedly Denathrius' wife I think? (the guy with
a talking weapon....) Naaru attacked there a long time ago too..
