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

[Alleo]

There's not much hand-waving in the lore of XIV, with the story having very few, if any, plot holes or inconsistencies that I'm aware of, so I am hesitant to conclude "well, time just conveniently flowed in such a way that Amdapor's founding and the Flood of Light coincided perfectly, then First-time virtually froze for 1,500 years, allowing for the events of the present to happen before the First can be rejoined. EVEN IF we assume this is the case, there's still something very, very strange going on.

Which would be quite strange indeed because Ardbert and his crew left after they realized what they have created which would then mean that they somehow waited so many years in stasis and then just came to our world and then the return was somehow really fast because the flood has still left one continent open.

(The story does say that different time goes in both direction, so theoretically 1 day on the first could have been x years on the source, but I am not a fan of that time stuff anyways...so much convenience)

[Crushnight]

Storge is believed to be a corrupted Sprite or Fairie and that it feeds on memories.

Thank you ^^

[ObsidianFire]

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.

However, since both of them are Voidsent, neither of them are really dead. They (and all the other Voidsent we fight) are really just possessing the bodies of whatever the mage who summoned them provided them with. Both Diabolos and Feridad are still alive in the Void waiting for some mage to summon them to the Source again so they can feast on aether.

Voidsent's forms (especially in the Source) are not their real forms at all and shouldn't be considered to be what the Voidsent really look like. Voidsent tend to like possessing specific kinds of objects and corpses... everything from suits of armor to eyeballs to gargoyles to people. As far as I know, we have never seen a person "transform" into a Voidsent because they were in an area that had "too much" Darkness. What usually happens is that a Voidsent kills a person and that body is immediately possessed by another Voidsent from the Void. You see this a lot in Haukke Manor (both normal and Hard). If anything, the closest we have seen to someone "transforming" into a Voidsent would be Nero in the World of Darkness raid. But even that doesn't literally make him into a Voidsent then and their. It instead infects his aether and he starts crystallizing in the Void.

Closer to the issue the OP brings up, there's like... three different proposed "causes" of the Flood of Light on the Source depending on which quest line you are talking about. The MSQ attributes the Flood of Light to Ardbert killing Mitron. The Role Quests attribute the Flood of Light to Ardbert getting his Crystal of Light. The 8-man Raid attributes both the Flood of Light and the origin of the Sin Eaters to Eden. So at this point... I think we actually need more info on what really did cause the Flood of Light. Because if it is Eden and Eden is what makes beings into Sin Eaters... How do we get Sin Eaters existing before Eden started turning other beings into Sin Eaters?

[YianKutku]

(Can time flow slower on the first? I don't remember if that was confirmed.)

Time can indeed flow slower on the First compared to the Source, as the Crystal Exarch explained. Sometimes the Source is faster, sometimes the First is faster.

However, assuming time only flows forward at whatever pace (even if it's effectively a time stop), the Amdapoori "seeing" a Sin Eater Forgiven Cruelty still requires some sort of future sight. Which can happen (see the list of prophecies extant in the FFXIV setting, eg Mezaya Thousand-Eyes and the Gerun Oracles), but it does seem kind of lazy narratively. ("It's magic, we don't have to explain it.")

Basically, the first appearance of Sin Eaters on the First, as seen by the denizens of the First, was about a hundred First years ago, after the Flood of Light's advance was stopped by Minfilia. The Flood of Light itself happened only slightly before that time, since Ardbert and co's adventures were also said to be about a hundred First years ago.

So the Shadowkeeper was "vanquished" (putting it as non-spoilery as possible) about a hundred First years ago, the Flood of Light happens, and Ardbert's crew sacrificed themselves to jump to the Source, where the 3.x patch MSQ happens. Minfilia joins Ardbert's crew on the return trip, returns to the First, where it is implied that not that much time has passed since the beginning of the Flood of Light. Minfilia stops the Flood of Light, Ardbert's crew minus Ardbert himself sacrifice their souls to help, and then Sin Eaters appear.

So the Sin Eaters would only have appeared after Minfilia went to the First, if judging by Source time. Which is obviously much later than the Fifth Astral Era time of Amdapor and Mhach and Nym, again going by Source time.

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.

[Atamis]

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.

I'll just repeat my previous post in tldr fashion since it seems it was ignored.
Sin Eaters can be a natural consequence of absorbing too much light aether. They were not necessarily invented on the first, but they are the first to suffer such on a cataclysmic scale. I would also argue voidsent are in the same position but being devoid of any aether well beyond simply draining the body of aethers.

So a completely valid explanation is Amdapor seen random sin eaters, maybe not from the first, maybe just in magic hammer space, and they copied it. I would assume getting something to become completely aspected to one aether is particularly hard, especially given cross contamination is possible. Maybe they didn't have the means to infuse something with pure light aether and instead chose to imbue a preformed block and statues with light aether to mimic it as best as they could.

I say sin eaters are a natural thing as the little boy in Amh Araeng in the Inn at Journey's End is slowly turning into a sin eater, it was not a forceful encounter. His aether was locked so only light could enter and everything else slowly seeps away or is being converted. Once anyone passes a thresh hold they are transformed into a sin eater. Normal people just wandering about don't chance because they can still regulate their aethers normally.

[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.

[TankHunter678]

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.

And given that Eden, a massive source of primordial Light, had been sleeping on the First then Sin Eaters would be able to form in its immediate vicinity, but until the light grew strong enough to reach across the world then the Sin Eaters would not be able to leave the isolated location that Eden was sleeping in.

[Zohar_Lahar]

Given that synthetic voidsent had been created in the past on the Source, it isn't too much of a stretch to think that synthetic sin eaters could be created. Even on Norvrandt, the Tank role quests showed an evil wizard was creating synthetic voidsent in Voeburt (some of which persisted even after the Flood of Light).

[Ouen]

... at any point in time an Ascian could have setup a vision for one of the Amdapori to let them peer through the barrier and see these Sin Eaters. The goal obviously being to give the Amdapori a means to take the fight to Mhachi and their voidsent. With open war being possible it sets the stage for the War of the Mages and the calamity it sparked to follow.

I considered that Ascians might have had a hand in the matter, but it's hard to draw further conclusions until we know precisely what capabilities Amdapor had so I didn't explore that idea very much. However, reading your reply, a question that now occurred to me is why would they use Sin Eaters in particular? How would this empower Amdapor and facilitate more chaos than if the Ascians had not intervened? What did Amdapor and the Ascians stand to gain from this transaction that they would not have otherwise?

What benefits does simply shaping the golems into the aesthetic form of a Sin Eater confer? Wouldn't normal stone golems (e.g., the Clay Effigies, Mana Idols, and Mana Pots encountered while descending into the Crystallum) suffice? They already had normal golems that were very capable of fighting, why would they need to create additional golems that were doubtless more difficult to make than Idols or Clay Beasts?

A possibility might be that much like the patterns that arcanists and geomancers utilize or the way that various weapons have divots and carvings, certain patterns enhance the flow of aether. Taking the form of Sin Eaters might allow for stronger golems. However, I find this a weak justification for all the effort it would take for the ascians to step in and teach the Amdapori about cross-dimensional beings just to make their golems, already functional, more effective.

On a side note, something curious to me is that none of the "Gargoyle" statues in Amdapor Keep are identical to the gargoyle-type Sin Eaters (Tesleen's model). In fact, they are actually quite different if you examine them. Here's an album I put together comparing some of them.

If those were indeed made in the image of the Sin Eaters, then they're not 1:1 like the Winged Lion or Kuribu recreations. They are golems, but interestingly they are identical to voidsent, and NOT to Sin Eaters. This may be an oversight, as they probably didn't have models of the First gargoyle things made yet.

Also, this probably has no significance, but I found it interesting that the zombified Amdapori White Mages you fight are named "Amdapori Gargoyles" while the adds that look like what we think of as gargoyles are called "Putto."

[kidalutz]

Okay so the thing is you got to look into the war of the magi for the answer as to why there were striking similarities between the guardian Angel Statue and the Sin Eater we killed in the first Shadowbringers dungeon. and really it has nothing to do with any time gazing shenanigans or pulled from the first hijinx. We already know that Sineaters were normalbeings that had thier Aether suddenly corrupted to pure light aspected aether thus causing transformations that eventually ended with a painful change into a full blown aether consuming sin eater.

Now during the war of the Magi Amdapor and Mhach were at war with each other as well as against Nym. One thing about war is it is the father of invention as the Mhachi were coming up with new and terrible ways to lay low thier enemies with demons they pulled from the void the Amdapori wielders of white magicks likely would have invented new ways to harness white magick including twisting light aether in ways that it simply wasnt meant to be Sineaters as we understand them would have surely been the result but without such a HUGE flood of light their damage would not have been as widely spread it was mentioned through the scholar quests that the Amdapori were not friendly to Nym and jealously held their white magicks in secret,I offer the supposition that it wasn't just the magicks they held and guarded but the Sineaters that were created through their experiments.