Revelations from Patch 7.4: Into the Mist

[Vallavia]

They seem to be setting up that The Key might be something sinister.

I would buy this, or alternately that it draws its power from or is composed of something of such tremendous power and esoterica (without being necessarily malicious) that it surpassed the knowledge and skills of even the Ancients. The accounts of it from Preservation, Alayla, and Miayli all paint a picture that the uses they know of for it have been either haphazard or very ignorant of how or why it works, like trying to grasp something far too large to hold. Halmarut's dialogue gave some shades of this too, particularly the allusions to the specific something that's creeping in ('nature's proclamation') in Hydaelyn and Zodiark's absence. I'd wager this is also what Miayli identifies as fate, which immediately called to mind the quote from Emet-Selch to commemorate Azem's crystal:

Though the world be sundered and our souls set adrift, where you walk, my dearest friend, fate shall surely follow.