That Yggdra's army was meant to start Ragnarok and become demons... But that seems... Unusual. Although the victor's write the history so 'humans' could've become 'demons. I mean. Riviera has no humans ya know?

I believe somewhere in Riviera, it mentions there are multiple "fallen" angels that resided in Tetyth because they sought greed and power. I speculate that one of those angels managed to survive and Become THE Archangel in Riviera. It's possible that the power of the Archangel is either blessed upon any other Angel like the "Grim Angel" Status, or could be obtained through embracing the power of sin, which the one in Tetyh seemed to do. Because of this, it's possible Marietta and the one in Riviera might be Twin Sisters, sort of like Aegina and Luciana are . (See, I told you my take is convoluted.) In my story, Marietta's Twin Sister slays her "Good" counterpart out of jealousy for her rank, is condemned along with the rest of the Angels, and obtains "Archangel" Status by embracing her personal sin of murder.

First of all, the island that rose to the heavens could be considered as Riviera. Even though it was smaller in YU when you were on it - but use your imagination here.

Ok, now, going with the 'bad' ending in YU, Marietta was supposedly killed by Yggdra and her army and Ragnarok had begun. The gods, startled by the strength of the Royal Army, consider them demons - for their inhuman ability to challenge the gods.

The war rages on, with no side seeing victory nearby in the future. This leads to the Gods creation of the Grim Angels - leading to the Gods destruction and their hand-down of power to the Seven Magi (let's consider them as a council before the Gods' deaths) - but even then, the Grim Angels alone are not able to fully turn the tide of the war. They are, however, able to push the Royal Army back to the island, back on Riviera, and keep the fight there.

The Seven Magi, seeing this as their only opportunity, send Riviera back down to Earth, yet not all the way.

The island, now in the borderline of heaven and Earth is the new battleground for the bloody Ragnarok. The Grim Angels are successful, and finally deal a heavy blow to the Royal Army. Though it is not enough for the Seven Magi, as they foolishly (insert powerful attack - something like an Ankh for example -) and destroy every living creature: Grim Angels and Royal Army alike on the island of Riviera.

Devastated by the amount of damage the war had cost, the Seven Magi wished to learn from the Gods mistake, and hope to prevent something like this would never happen again. So as their first act of supremacy, the Magi (and this is where it gets even crazier) destroy every living being on Earth and create the Sprites - the human equivalents - on Riviera. This way, the Magi could monitor the Sprites and form them into being the Gods before them had intentionally wanted before reforming Earth. Yet something had been miscalculated. The essence of the dead Grim Angels (forgot what they were called >.<) had seeped into the soils of Riviera. The first several hundred years go by unnoticed, but soon, changes begin to occur. Sprites are beginning to deform from random contacts of the essences of the Grim Angels that had fused to the floating island.

The rise of demons...

This goes unnoticed by the angels up in Heaven.

Now Hades, after the destruction of the humans on Earth, soon discovers Riviera, and make his home deep within the floating isle, shielded by the mystical powers of the deceased Grim Angels, the Magi don't suspect a thing. Hades is able to control the newly formed demons by creating Greater Demons - Accursed. This is where the revival of Marietta comes to play, as she was killed on the island. Though still controlled by Hades, she still keeps her faith by declaring she is a fallen angel, yet has no power to stand up against the powerful Hades and (let's just guess she's controlled like a puppet.)

You know, just because Ending C has Yggdra and co. starting Ragnarok doesn't mean that it's THE Ragnarok we see in Riviera. My personal theory is that YU would end in the "canonical" A ending, but that Ragnarok would eventually be started by demons some time afterwards (from hundreds to thousands of years, we don't know). Marietta, the young angel we see in YU, would be kicked out of Asgard at around this time.

Asgard, Utgard, and Heaven's Gate are all floating islands--Ancardia was originally a floating island but was expelled to Midgard (the human world) as we notice.

Midgard is where the humans live; my guess is that after Ragnarok, the Sprites (such as the Undines we see in YU) got moved to Utgard/Riviera to avoid conflict with humans. Remember, the Sprites were enslaved by the demons and fought for them in Ragnarok.

You can see this stuff a lot more coherently in The Tainted, where the story of YU gets referenced a few times by Celina, but yeah. Sorry if that made no sense.

Basically I think YU and Riviera take place in the same universe, but different places and different eras. 'Cause there are just WAY too many Riviera references in YU, from Banangos and "kokori no mi" (translated in YU as "Kokorinut" while it's "Applecot" in Riviera) to Mjollnir (which they say is a Diviner) to Nessiah's design (you just know that it ain't coincidental that he looks so much like the Magi...) and Marietta's cameo at the end, for them to be just references.

And since we don't know the life span of angels, the Marietta we see in YU could easily be the same archangel in Riviera or some relation of hers.

Marietta is a Fallen Angel?