[Spoiler: 7.0] I can't stop thinking about this.

Why does Azem's crystal symbol appear on the key AFTER we use it in the final fight?

After talking to a couple ppl who have also finished MSQ I am guessing that maybe another shard of Azem created the key to explore other worlds or even possibly Azem himself helped in its creation.

But why does it react to some people versus others? (E.g Sareel Ja held it but didn't react until it was in Zoraal Ja's hand.) What made him "worthy"?

My guess is that his soul/will with strong enough to activate it. Sphene says only the key can open it (she can't open it from her side). it's possible the key reacted to Zoraal Ja's strong will/aether, and stablished a connection. Sphene even said later that it answer to only fevent desires.

Granted she herself could also only use a partial part of it powers because that's all the preservation could figure out even after all those years with it.

But what powers now did we unlock in the key now that we have it. I'm super excited to see where we go with it.