To my memory it was when the inspiration for his airship / manacutter's name was revealed, which I believe was post-Weeping City of Mhach.
But you have the right of it.
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In fairness, I did have to look it up in the lorebook myself, because SannaR is right that is has been way too long since the Shadows Of Mhach questline. If you challenged me before this sidequest to name even one other member of Leofard's crew (apart from Cait Sith) within thirty seconds, I'd have failed that challenge. (Stacia the ship's purser and Utata the ship's engineer, incidentally.)
I had a moment of "Leofard is adopted, which is why... wait, was he adopted? When did we learn this? Could he be another mixed race like Hilda? But he doesn't have pointy Elezen ears... oh, right, the lorebook! That should have the information I need!"
Heh. I didn't have access to the 1st one when I was doing the quest and didn't think to look it up on line. So when I did it it hit me like Mikoto's future sight popping up in Bozja and making me feel as though I had missed something.
Somebody on Reddit posted an interesting idea on where the story may go with Golbez.
He may be trying to make the Moon itself drop onto the 13th in a "collision" that will force the light aspected aether of the Moon to fuse with the void itself, making a part of the 13th stabilize like Norvrandt, a small "safe zone" from the endless darkness. And with the scene of the Watcher and the Moon at the end of the story, it gives this idea some merit.
Noodles and oodles and boodles of characters
I'm not 100% that, that was a Watcher. Robe appear white/light, so it could be, but the energy around him looked reminiscent of the Azem sigil magic. I like the theory you're sharing, but I think other options are just as likely. Though the ones I can think of aren't nearly as fun. (Maybe he confronted Elidibus when Elidibus came to retrieve Cylva/Unuk)
There's a few lines also mentioned...
Golbez mentions his "Prison of Passivity", and looks up and reaches towards the sky, almost as if to the Moon. He had also mentioned that the "gates" were finally opened. Part of the reason why I'm pretty sure it was the Watcher, was because of the background of the scene. Similar enough to almost guarantee it.
https://cdn.discordapp.com/attachmen...86/unknown.png
https://img2.finalfantasyxiv.com/acc...447a9a19ba.jpg
EDIT: Trying to fix pictures, give me a moment.
EDIT2: Fixed it!
I don't think there's enough detail to make a definitive identification.
The person looks pretty small but even if he's a voidsent in this universe, Golbez is a really big dude. Like, the Garlean royals' armor appears to have taken a lot of inspiration from his own, especially Varis, and he was friggin' giant compared to normal people. Though we don't have a fr... no, we do, with the scenes where he's talking to pre-empowered Scarmiglione and Barbariccia. He's still hueg.
Using the moon to rebalance the Void is... an interesting idea. Gotta wonder if the Zodiark bit that should still be sealed up there is, though... might end up backfiring.
Character Limits ahoy
I believe Yoshi P has confirmed all the bits of Zodiark are gone, so that should be a hollow moon now.