I have theories that The Key is a lot more dangerous then we think. I think it might actually be Auracite. Endless Sphene did say it 'responds to fervent desire.' My tinfoil hat theory is that the reason why it had Azem's sigil on it is because they had to deal with its threat, but couldn't completely destroy it, and had put a protective seal on ages ago. The reason why no one has been able to fully unlock its power is because of the seal. The Pandaemonium raids did indicate that Auracite had found its way to the star even during the time of The Ancients. It also begs the question: why would pre-sundered Azem need a device for dimensional travel, and potentially dimensional fusion, undoing Venat's sundering?

I won't be surprised that it's been tugging at Y'Shtola's desire for shard travel, and that's why she's been insistent on wanting to take a look at it. It would be careless to leave something so potentially dangerous with someone we care about.