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

[ICountFrom0]

Diabolos' history in the Source is well documented. He's a high-ranking Voidsent summoned by Mach to unleash on Amdapoor near the end of the 5th Astral Era. The Amdapoori succeeded in sealing him up until you breach his wards at which point he leaves Amdapoor. The same thing goes for Feridad, the last boss of Amdappor Keep (Hard). Later, they would both turn up in Dun Scaith. Feridad would finally be defeated there and Diabolos would absorb Scath's power and be defeated.

Which now makes me wonder, was Diabolos originally a Darkwarden?

He was orriginally a hero of the 13th. But he ate primals until he corrupted.

Therefore, the possibilities that allow Amdapor to see a Forgiven Cruelty Sin Eater would be as follows:

- Future Sight, as mentioned.

- 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.)

- Time can flow backwards relative to Source and Shard.

Or, the production of sin eaters is a "natural science" part of the under pinning of reality. If you get too much darkness you turn into a voidsent. If you turn too much light, you turn sin eater. Could the other less primal elements also have effects at the absolutes? Maybe.