Old Man is the god who lives on the Moon (Grallin I think he was called by Apsalar in GotM). Why do I say this? Because he's got a BIG SHINY MOON FLOATING BEHIND HIM!

Okay, admittedly, it doesn't explicitly say that, but first there's the cold airlessness, and then the object behind him is described thusly:
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Come on, none of you tried to reach up to the moon as kids?

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And, of course, the moon but reflects the light of the sun.
The child who approaches which the Old Man is waiting for could either be the actual moon, or perhaps the world of Wu itself. Admittedly, I'm more hazy on that part than the identity of Old Man.
As a side note, I quite enjoyed FoD. I do somewhat agree with the opinions that it petered out, but I enjoyed it because it was not at all what I expected. I wasn't much looking forward to this new series, as I didn't see the point of just expanding a bit on the events we already heard about and knew, but the way everything was turned upside down, and the mythos shown to be exactly that, blew me away. once again with SE, nothing was quite as it seemed
