There is a different artifact or set of artifacts for every single spec coming in Legion. There are several 2-handed swords (two of which are a pair, for Fury Warriors,) a 2-handed axe, a polearm, a gun, a bow, a few sets of daggers, some fist weapons, several swords that are either paired with shields, off-hands, or just other swords. And of course, there are a ton of staves.
And then there's Skull of the Man'ari.
The Skull is a main-hand weapon - your "main" artifact will not be your off-hand item (I think only the shield-bearing tanks really focus on what's equipped to their left hands.)
But the Skull is not going to be on the end of some stick to serve as a staff or a wand. There's no off-hand item that Demonology Warlocks will be using, as far as I can tell.
Instead, Demonology will simply walk around with a floating skull nearby, rather than carry any sort of weapon. I have absolutely no idea what their melee autoattack animation is going to look like, but given that my Warlock is pretty happily demo-specced (even if the spec is getting utterly transformed, I expect I'll stick with it,) I'll find out soon enough.
I've been looking at some of the alternate models for the Skull, one of which is just a full Eredar head that floats around - basically a Draenei but with a few spikes where eyebrows should go, and the option to go with colors other than blue.
It's really bizarre, and just the sort of morbid touch you'd expect from a Warlock. I wonder if the Skull will talk to us, like our constant buddy while we fight our way across the Broken Isles.
As far as I know, this is really the only artifact weapon where they've gone so completely outside the box. It's actually a bit disappointing that there hasn't been more variation in artifact types. All three 2-hander wielding (actually, all four, given that Fury is going Titan's Grip) strength classes are using swords. It's actually kind of unfortunate that the Ashbringer is so obvious as the Retribution artifact, as it would have been nice to give them a nice blunt warhammer like the one Uther wielded. We're not seeing any one-handed axes or two-handed maces, and I think the closest thing we're getting to Crossbows is a pair of alternate skins for the Beast Mastery Gun and the Marksmanship Bow.
We are getting a new weapon type for Demon Hunters so that both specs can use Glaives (which I guess are a cross between swords and fist weapons.) But the Skull of the Man'ari is truly unlike anything we've ever seen in-game.
Especially given the artifact system, I almost wonder what they might have come up with going further outside the box. You could introduce a flail, for example, or give one of the hunter specs a pair of pistols like Lord Godfrey. The artifact system makes it so that you don't necessarily have to keep bringing these item types back in the future.
Anyway, given that Demonolgy's already going to be summoning their regular minions as well as wild imps and fel stalkers, it's kind of amusing that among their demonic menagerie will be a floating demon head who will probably be second-guessing your every move.