I'm reticent to put forward any proper theories, in part because the questions being raised feel far more interesting if they're not the easy answers.

I don't think the key makes sense to be an Ancient creation, for simple reason that the shards and the Ancients by nature didn't exist simultaneously. The key would've logically done absolute crap-all in the Ancient world; if it was intended for terrestrial transit and just got jacked the hell up by the sundering somehow, then it wouldn't have even been useful because they already have aetherytes (and unlike present-day people, everyone seems readily able to use them). However, it would make absolute sense that the key to travel between worlds would invoke a person with the actual title of 'the traveler'.

That implies that someone post-Sundering knew who Azem was enough to invoke their title and history--which itself would be interesting, because everything we've been told suggests that the knowledge of Amaurot was lost to vague dreams near-immediately. The key itself doesn't resemble any of the works of civilizations we've met; it's closest to Amaurot, but only in that they're the guys that tended to do a fair amount of twists and spirals. Even that's not quite right; compare the key and its internal crystal to the closest comparison we have, Elpis' aetherytes, and you'll see that the key is entirely different. The key twists in its structure, while the Ancients only have spiraling ornamentations.

So yeah, my best theory on this is that we don't have the answer right now. And I find that far more interesting than the notion that we already do.