Well first and foremost there was only one harbinger and his name was Kodlak.
Now with that said, I get it, Xal’atath is ni e, character evolved a lot since Legion and I love how in BFA she blames the “previous owner” for not having her true potential, love her shenanigans, she is great, all that…
BUT everyone who actually played Legion know the most epic sentient weapon was the Skull of the Man’ar (Warlock OH).
So… question… we will ever see it again?! Maybe a plot like Xal’atath?!
Snarky Aluneth would like a word with you on that. He had some great lines. I’m pretty sure the skull just grunted and drooled at me.
Good call…
So Blizz, two nice characters to mess around with.
Don’t get me wrong, I did enjoy my grunting, drooling severed head. But Aluneth was more amusing. lol
Maybe you might enjoy the dwemer aesthetic of the Earthen. 
Xal is, best as we can tell, literally an old god who was imprisoned by the other now defeated old gods. I don’t think your skull is in the same league.
Its ON!
Skull > Xal’blahblah
When did the skull get to be the big bad in its own expansion?
Skull is just a Man’ari backstabbed by his eviler than thou student Archimonde to impress the Eredar Leaders so that he could become one as much as I can tell.
Archimonde most certainly surpassed his mentor ages ago!
Xal’atath and Aluneth are on a much higher league than that lowly Skull.
I wouldn’t worry about an harbinger I’ll be worried about what comes afterwards.