With the rift between worlds open, various demons from the other world/dimension were spilling through. This is where two heroes - Yunalesca and Zeion - came forth, used their powers, and sealed the portal. This can be further speculated in a news article located at thegia.com, right here. Note that the seal wasn't a permanent seal; they didn't know how to permanently close the portal and return Sin to their homeworld, so they merely rushed to close the portal. It served its purpose; nothing could pass through the rift between worlds, but it was laying dormant... waiting to be reopened, someday.

The demons, known as Sin, were formless. Basically, intangible; they were like a force of nature. You truly can't attack something that you can't see. Sin was attracted to technology, and would attack it. In fact, the best way to describe Sin is to think of poltergeists: formless ghosts with the power to create all sorts of havoc. Sin is, very much, the same thing.

The scientists who were originally involved in the initial portal opening (though no one knows about that but themselves) created the Ebon religion; oppose technology, or face the threat of Sin. People rejected their philosophy at first, but soon learned the lesson from the invisible demons. Thus, people flocked to join the Ebon religion, and the group of scientists-turned-priests became some of the most powerful and influential people on Spira. So in the end, the Ebon religion formed, preached against, and gained power from the mistake that tumbled Spira into the dark age of technology. The original tale of the occurance was only passed down to the most elite, powerful members of the religion, so they could never forget the truth.

The truth had a terrible price, though. Throughout the years, Sin would mentally tempt the leaders of Ebon, to reopen the gate so that more demons/Sin could enter Spira. Through strong mental conditioning, the leaders of the religion would fight off their attempts. Besides, it was impossible for them to reopen the rift: Yunalesca and Zeion were the ones who originally closed it, and therefore, would be the only ones who could reopen it. Of course, they were long dead. So, Sin would challenge the priests, and the priests would always prevail: until the present.

One of the elite members of Ebon, Seymour, finally fell prey to the power of Sin. He was obssessed with finding a way to reopen the rift; of course, he kept this obssession to himself. He acted perfectly normal, as a leader of his clan and leader of the religion, and tried using his vast resources to find some way to reopen the gate.

Eventually, Seymour meets a young summoner named Yuna. Yuna, Yunalesca. Coincidence? Seymour doesn't think so. Seymour reveals the story of Yunalesca (obviously leaving out some key parts ^_~) and convinces Yuna to marry him. This should occur sometime into Tide and Yuna's quest in FFX; Tide's attempt to found out about & destroy Sin, and Yuna's attempt to find the "Ultimate Summoner" (which is talked about on the main FFX page at FF:WA, right here).

After some time, Seymour sees Tide and company are getting too close to the truth (hey, they're going around Spira trying to figure it out anyways, y'know?). So, he sets his master plan into effect. He wounds Yuna's bodyguard, Kimari (wound or kill, though, if he dies, there will be some way to resurrect him in a side-quest) and kidnaps Yuna, taking her down to the secret location undersea where the original rift was located.

One action-packed FMV later, Yuna is forced to use her power to open the seal. Being a descendant of Yunalesca, Yunalesca's sceptre had been passed down throughout the many, many years: and now, it's Yuna's sceptre (remember seeing it in the various screenshots/FMVs that have been released so far?). That very same sceptre, combined with Yuna's summoning power, was what was required to reopen the rift. With Seymour forcing her, she has no choice; and so, the rift was reopened. Spira becomes overflown with Sin. Tide and company arrive to save her, but albeit, a tad too late. To add insult to injury, Seymour not only destroys the septor, meaning it's impossible to close the gate, but jumps into the gate, venturing to the demon-world!

So now, Spira is being flooded with invisible demons who are really messing up the planet's ecosystem with various storms, hurricanes, earthquakes, tornados, typhoons, etc. Yuna ventures to visit the Elder Summoners...think of it as the Chief Summoners. The big shots. ;)

They reveal their own version of a prophecy, which involves the one who opened the rift having to venture to the demon world and shut it down there. While they're sceptical, they agree.

So, now, the entire party ventures to the demon world (while this won't be its name, let's just call it "Sinworld" for simplicity's sake). Note that the demons on Sinworld aren't intangible and invisible; they're very much flesh and blood, and easy to beat to a pulp. Spira, though, something happens to them and they become invisible.

Yada yada yada, they venture through the demon world a bit, and eventually come to some sort of demon fortress; residence of the Final Dungeon. They travel through the final dungeon, where they come face-to-face with Seymour! Duh duh duh!

One boss fight later, Seymour's defeated. The head demon of Sin, one big whopper of a demon (the fortress is his castle, anyhow) fights the group; he gets defeated as well. In a last-ditch attempt, Seymour and the demon merge to form the final boss, and, naturally, is defeated as well.

With the main demon killed, the forces of Sin fall apart, and the gate begins to fluctuate. Everything from Sinworld that originated from Spira is sent back there; everything from Spira that originated in Sinworld is sent back there. So the characters return to their planet, and Sin returns to Sinworld, and the rift between their worlds is forever sealed.

Seymour, freed from the Sin mind control, is mortally wounded. He apologizes, says his goodbyes, yada yada, then bites the big green burrito of death.

Note how this corresponds to the prophecy:

Dead for a thousand years...
(Portal closed)
A great terror arose from the deep.
(Sin possessing Seymour)
It swept across the world
(Various attacks)
A long past still lingers.
(Prophecy of Yunalesca/Zeion originally closing the portal)
Destiny cannot be denied
(Yuna is destined to one day reopen the rift)
A burder grows heavier.
(Seymour kidnaps her as he's possessed by Sin - the burden)
To the kingdom of the sun
(Sinworld)
Across the vast sea.
(Metaphor, symbolising time and space - Sinworld is a different world/dimension)
To the unknown
(Again, Sinworld)
The journey home begins.
(The journey to Sinworld/completing their destinies)

Tada! There's your game of FF10. Obviously, there's some filler in-between, and the filler is clichériffic. Such as:

- The Aeons (summon beasts) live on a part of Sinworld, completely seperated from the forces of Sin. The Summoner's power enables the seal to be broken while Aeons are summoned, just like Yuna could break the seal of the rift that enabled Sin to venture to Spira.

- Tide is a descendant of Zeion, and Yuna is a descendant of Yunalesca. Just like their descendants fell in love, so do they. The obligatory main-character love sequence will occur with Tide/Yuna. It always does. ;)

- The forces of Sin, while finally completely unleashed on Spira, will decimate the Ebon religion.

- Seymour will be related to someone, somehow. Perhaps Lulu? As well, Kimari will discover he's part-Aeon somehow. Perhaps an Aeon and a Summoner got jiggy wit' it and produced him? (Before you go "Ewwww!", let's not forget Maduin and Madonna from FF6, who produced Terra. ^_~)