Something doesn't add up regarding the connection between Amdapor and Norvrandt...

[Seraphor]

I find it hard to believe Sineaters never existed until a few weeks ago, relative to the source.

Sineaters weren’t an invention conceived at the Flood on the First, they’re a ‘natural’ consequence of an overabundance of unaspected umbral aether.
It’s not inconceivable that such circumstances could have happened at some point, on some world, that could have resulted in Sineaters, without going to such lengths as an entire world-consuming flood of light.

Or alternatively, to go by kidalutz's theory that Amdapoor created their own Sineaters. It seems conceivable to me that they could initiate a transformation into a Sineater via their light aspected magicks, but, without the background of a flood of light to fuel such a being, the stasis aspect of light aether simply runs its course, and the created 'Sineaters' they produced simply froze.
Perhaps they created Sineaters, but these Sineaters immediately petrified and crumbled, and this resulted in a sort of rapture mythos, that a person who was transformed in this way was pure of heart and somehow whisked off to a heaven somewhere, producing this sort of reverence. Therefore, the statues depicting this, and magicks to attempt to 'bring them back' to return those called to heaven, to fight against the dark forces of Mhach arose.