Demonology gets perhaps the weirdest - no, definitely the weirdest - artifact weapon. While the Skull of the Man'ari is classified as an off-hand item (it comes with a dagger, but to show you which is the important part, the skull's the one with the spellpower on it.
Something that most people have heard about is how Thal'kiel (the Man'ari Eredar whose skull it is/was) occassionally comments on things around you. He seems to have commentary on one boss per dungeon, and also sometimes just cries out stuff in battle like "fight you pathetic imps!" This isn't unique to the Skull - apparently Aluneth (Arcane Mages,) T'uure (Holy Priests,) and Xal'atath (Shadow Priests,) all being sentient weapons or sentient beings bound to weapons, will sometimes comment on your situation (though I haven't gotten any of those classes/specs up - I'll eventually see the Shadow Priest's stuff.)
But it turns out that Demonology goes a step further. While questing in the Broken Isles, your Felguard (assuming you have him out, which you should, as he does fine tanking for you while soloing thanks to Threatening Presence) will also sometimes comment on your situation - complaining about wandering around some forest (Val'sharah) or how the Vrykul are not such great warriors as they claim to be (Stormheim.) Thal'kiel will then respond to these comments.
This kind of blew me away, because my warlock has been hanging out with Felguard Zurilshoken since Wrath of the Lich King (ok, with the Wrathguard Khil-Barash for a while too,) and all of a sudden my chief minion has something to say other than the typical canned emotes.
Guys, Legion is awesome.