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

[geekgirl101]

Sin Eaters somehow existed on the First when the Flood of Light began, but declined to show themselves to the general populace for some reason. And at that time, the passage of time on the First was effectively a time stop while thousands of years passed on the Source. (This contradicts out-of-game lore interviews given prior to Shadowbringers, ie that a "proper" Flood of Light is just blank nothingness, but those could always have been retconned.)

Since sin eaters remain present where there is a strong Light source being emitted from a Lightwarden then there must be a point somewhere where there's a strong primordial Light presence for them to congregate, like a Bermuda Triangle where Light is at its strongest, and are unable to move beyond that point unless Light becomes more dominant on the rest of the star. This could explain why they weren't seen before by the inhabitants on the First. Emet-Selch summoned one to alter Vauthry before he was born so they must already exist somewhere. Like some have stated it's probably then a natural occurrence where overwhelming primordial Light aether warps living beings into Lightwardens and sin eaters. I think Thancred said something along those lines too when he ventured into the empty that if we remained in it for too long then the overwhelming Light aether would eventually turn us into a sin eater.