OK, before I begin, you need to brush up on the lores of Nautilus, Fizz, Nami, and to a lesser extent Kassadin, and remember that Nautilus's incident happened on the Ionian side, not the Shadow Isles side.

The Void has already arrived and is taking over the oceans of Runeterra. The black oily substance is a particular strain of Void creature. We know that prolonged exposure to the Void causes changes thanks to Kassadin. We also know that Nautilus was underwater a long while-long enough that he could find no trace of his former life, so he was exposed for a long while. He blacked out, so we don't know how long he was wandering, but... (Nautilus is also Void now IMO).

In order to get an abyssal pearl, Nami had to fight untold horrors for 6 days (I have another theory about this as well, but later). Untold horrors in a dark abyss? That sounds like Void to me.

Fizz's home was destroyed-thoroughly obliterated is what it sounds like, given that there were no clues about what happened to his people, and ruins were mentioned in the lore. Since I'm assuming that he didn't leave for more than a few days tops, something must have mounted an all out assault on a supposedly impenetrable fortress... that's underwater, which rules out normal forces. Since we are assuming that the Marai would not have the manpower to do so, and Demacia/Noxus/Ionia/Bandle City/Zaun/Pilltover wouldn't know about/GAF about a city that's underwater and protected by a mountain, that leaves the Void, who have motive, and obviously opportunity.

Also, both of the underwater races have/had a protection system. The Marai have the moonstone, and Fizz's people had the mountain fortress. YOU DON'T GET DEFENSES IF THEY AREN'T NEEDED. There are obviously huge threats underwater, and the Void is it.

Also, the Solari are in league with the Void. Or at least the senior members are. Sometime in the last 100 years, they purged the moon worshiping sect (Lunari), and removed all records of them. The Lunari are the ones who brought the moonstone for Nami's people.

I'm not 100% sure what the Lunari wanted with an abyssal pearl, but eh. It fits on as a tangent to my point.

Holes that you can help me sew up?

I'd say this is a reasonable theory, actually. I always thought Nautilus had been 'tainted' somehow, though I didn't realize it wasn't the Shadow Isles' doing.
It fits well with Fizz and Nami too, especially Nami. Cool.

I just don't see how the Solari would have anything to do with it. I think that was probably just an inner feud/ploy for power, which led to an unfortunate circumstance for Nami. You know how the Rakkor are. I doubt they would care for the Void, other than to fight it for glory's sake (and they already have plenty of enemies for that purpose).

I believe Nautilus armor is not a Void Champ, Rather a technology made by a highly advanced lost ancient civilization who accidentally discovered The Void during a research of space-time continuum.

Nautilus armor would probably be one of the mass-produced(Aqua-Variant recon-type) sentinels that acts as defensive line during that time.

His System went clunky during the infestation and he get sucked into a void hole(He probably thought he was in a slumber, but i think he was freezed in time, since time in the Void Realm is non-existent). Now That worm-holes(Inter-dimensional space holes).He became active again, He got lucky he got spewed out (probably just milleseconds just passed after he got sucked). but after he went back, Millions of Years has passed, everything has changed, people now are using so-called magic. Probably learned by studying void creatures during the apocalypse.

So basically, Runeterra currently is in a post-apocalyptic state after the Void Creature infestation.

Just using my imagination LOL.

Also, Nautilus Armor Became Fixed maybe because of the Atmosphere inside the Void Realm is Toxic(remember kassadin? he is using a mask with some kind of air support in his back, probably because his lungs are already damaged. and he is having a hard time breathing)

Dont be silly...Naut has been infected by Dark Water not the Void.

have you read nautilus's lore?
Once, Nautilus was a sailor commissioned by the Institute of War to explore the uncharted reaches of the Guardian's Sea. This expedition took him deep into unknown waters where he and his crew found a vast section of black oozing liquid that none of the crew could identify. Though their job was to investigate anything new that they found, no man aboard was willing to brave the murk except Nautilus. Only moments after he donned the hulking diver's suit and climbed over the ship's rail, something lurking in the muck grabbed hold of him. He clung to the side of the ship, but the thing below pulled him fiercely, rocking the entire ship. The other sailors grew afraid and made a terrible decision. As he stared and pled for help, they wrenched his grip free of the rail. He tumbled into the ink, grabbing the anchor in futile desperation. Dark tendrils enveloped him and he could do nothing but watch as the dimming outline of his ship faded away. Then everything went black.

When Nautilus awoke, he was something... different. The great iron suit had become a seamless shell around him, concealing whatever awful truth lay inside. All the details of his memory seemed fuzzy and indistinct but one fact remained clear: he was left here, alone in the sunless depths, to die. In his hands he still clutched the anchor that belonged to the men who had condemned him. Having no other purpose, he took this clue and trudged - too heavy to swim or run - in search of answers. He wandered without direction or sense of passing time in what felt like an eternal dream. By the time he stumbled upon the shores of Bilgewater, he could find no traces of the man he was. No house, no family, no life to which he could return. Terrified sailors who'd heard his tale directed Nautilus back to the Institute, but the summoners refused to relinquish the names of the others they commissioned. By then Nautilus had learned about the League of Legends and there he saw an opportunity to discover and punish those responsible for the time and life he lost.

''When consumed by utter darkness, there is nothing left but forward.''
-- Nautilus.

While the untold horrors of the deep could match the same description of Void creatures, it's been mildly suggested that Nautilus has more in common with Varus. Notably, the intensification of negative emotion of revenge.

There is more evidence to suggest, but I would have to do a bit of digging.

When Fizz finally reached land, he came to Bilgewater, which is firmly on the side of the Guardian's Sea. I doubt Fizz had somehow passed from one ocean to the other, so... Fizz's home was in the Guardian's Sea? Considering Nautilus got corrupted in the Guardian's Sea, if the Void is anywhere in the seas it's likely to be in the Guardian's Sea.

The goop in Naut and Varus' lore seem like the same thing to me, with similar properties.

If they release an aquatic themed skin for Kha'Zix, then I'll be rather suspicious that this is actually true.

If they release an aquatic themed skin for Kha'Zix, then I will definitely be suspicous that this is true.