I had read that you get an artifact off-hand for free when you hit level 70, but by the time I actually hit level 70 I had kind of forgotten about it again. Tonight I hit 70 on my great weapon fighter and suddenly remembered.
Several sources said that you simply get it from a quest from Sergeant Knox. Very straightforward, right? It really should be... unfortunately my great weapon fighter had hardly done any quests on her journey to 60, so talking to Sergeant Knox presented me with a list of about a dozen quests, most of them telling me to go to different low-level zones. Which one's the right one?
The answer is: the one called "Protector's Emissary". You're welcome.