If the amulet of kings is crushed, then wouldn't that mean Oblivion can still attack Tamriel?

Oblivion could only attack Tamriel because it was weakening as a plane of existence. Places like White-Gold Tower and Red Mountain fortify existence, but only under certain conditions. Those who wear the Amulet can uphold a pact with the gods. This interacts with White Gold Tower to keep the Daedra out, even though that was not the original purpose of either the tower or the amulet. Martin turned into Akatosh, so it is safe to say that the Covenant with the gods is intact and no longer reliant on the symbol of the Dragonfires and the wearing of the Amulet.

Thank you very much

That brings up a question - how then were Meridia's Aurorans still able to enter Mundus?

The Addon "Knights of the Nine" may not be the best Addon, Bethesda ever made, but I think, that the Lore, which it brings in, is more important than the Lore of the "Shivering-Isles"-Addon. A few months before Oblivion was released, the Nu-Mantia Intercept appeared in the Bethesda forums. Nu Hatta told us a lot about towers and stones and he warned us about a returning of the Ayleid. Short after the Oblivion crisis, an mighty and powerful Ayleid return and we've got to beat him.

How Umaril and his followers, the Aurorans, entered Mundus is not known yet, but I assume, that the Ayleid gonna be a important part of the story of TES V. I don't think, that they'll invading Tamriel, but they will be a part of the story.

The same reason why Hircine can enter Mundus in Bloodmoon, it was foretold.

The Aurorans can enter Nirn in the same way that Deadra can still enter Nirn, they were summoned, thier bodies disappear just like summoned creatures. Umaril was an Ayleid, not a Deadric Prince. It is still possible to open doors into Oblivion but not by force, I think that was how the 'strange door' was explained, anyway.

The door to the Shivering Isles was explained by Haskill wasn't it? Didn't he say it was not a threat but an invitation

It's my theory that Umaril waited patiently until around the time Uriel Septim was assassinated and entered the Mundus either just before the events of Oblivion or during the main quest. He slipped quietly through a portal into Tamriel amidst the chaos of gates opening everywhere, and hid in an Ayleid ruin. Such an act would have been completely unnoticed even during peaceful times, if done under cover of darkness in a remote area. Considering the pandamoneum caused by the Oblivion Crisis, it's easy to see how he could have slipped right in and remained hidden.

The story works no matter what stage of the main quest you're on. You can't start playing without Uriel being dead, so the second you step out of the dungeon and head for Anvil, the barrier between Oblivion and Mundus is wide open, meaning he could enter.

Even if you beat the main quest and then go to Anvil, you could say he slipped in while the gates were open.

It works either way.

It is however safe to say that all this is just one opinion. And while it's all well-supported and sounds real buttery, we'll probably remain mostly ignorant for time being. But good point Hairdo.

*shrugs* True, it's unsupported but very logical, and the devs likely expected us to come to that sort of conclusion. The conclusion that he came in while the gates were actually open, and not after, that is - regardless of when the player embarks upon the quest.

Your theory is nice, but notice, that there are many different Oblivion stages. In TES IV we entered the realms of Mehrunes Dagon, the Deadlands and Daedra entered Mundus off the Deadlands. But there is a different realm - the waters of Oblivion, were Daedra get reborn. And only there Umaril could have stayed (or maybe in Meridia's realm). But in fact, there is no chance, Umaril could have been in Dagons realm and passed an Oblivion gate to Mundus.

what?
Barriers are down, Dagon's not the only one who could have opened gates, though I'd imagine he's the only who wanted to launch a full scale invasion.

Yup. In addition, the realms of Oblivion are not protected from Oblivion in the same way Mundus is. To get to the Battlespire Mehrunes actually marched through half a dozen other realms. Using one of Dagons gates should be no problem. If he didn't force his way in, he might have bartered for it.

But anway:

That's a point, but do you really believe, that there have been other Oblivion gates without our knowing? We have two types of gates seen yet: the "http://www.pibweb.com/Review/images/oblivion_gate.jpg" and the "http://upload.wikimedia.org/wikipedia/en/b/b8/The_Elder_Scrolls_IV-Shivering_Isles-gate.jpg". In this case, we can assume, that every gate to a specific realm has a specific look. If there has been a "new-looking" gate, we (the CoC) probably would have known.

Not if it was open for a very short time. It would only need to be opened for a few seconds to allow Umaril to pass through, and then he could summon his Auroran legions without a gate. Also, who's to say he didn't come in through a Deadland gate? Or perhaps a gate within Garlas Malatar itself?

Who says gates have to be opened in the middle of the country side? If martin can open the portal to paradise inside cloud ruler, I think a daedra could open a gate inside an old ayleid city.
And no, we probably wouldn't have known, 'cause the gate probably opened in some out of the way place that no one has any reason to go. It's like Mora's shrine in oblivion, you got absolutely no friggin reason to even go near the place.

I agree - and what more appropriate place to open that gate than Garlas Malatar?

From a lore perspective, Dagon wanted his minions to be able to spread out quickly and do maximum damage (which is why when his servant Mankar Camoran lost the amulet, he switched tactics and focused on the Imperial City instead). From a gameplay perspective, gates are easier to find outdoors rather than searching every cave, fort, ruin for the gates.

its also easier to move an army through an open field then the cramped death traps that dot cyrodiil.

Um...yeah. I believe Umaril opened a gate to his Realm in Garlas Malatar, and held it open long enough to move a massive army of Aurorans into the ruin. Since his spirit was bound to the realm of Meridia and she lent him some of his powers, he probably directly opened portals in the Chapels to send Aurorans in and attack them as well. It makes more sense than them marching into town, through the gate and past the guards and citizens in broad daylight.

Especially since neither the guard nor any citizens saw a thing; it would be believable had it been a lone, nondescript assailant, but knowing as we do that it was the Aldmer-Daedra known as the Aurorans, a single one of which would have raised eyebrows and a squadron of which would have caused alarms to sound, I think it's fair to say that a portal was opened within at least the Anvil chapel. An alternative possibility is that Umaril teleported them there himself. As a thousands-years-old Sorcerer he would most likely have such power at his command.

It appears that the Aurorans may have been summoned in the regular, conjuration-related way. Hence why they were able to suddenly appear and why they disappeared after their jobs were done.

I've got a question back to topic. In one of the german forums, someone told an interessting thing. When I read it first, I was impressed and took it into my lore knowledge. Now I know more about the lore and it sounds not so realistic as it did, when I first read it. It was a news about http://www.gamesas.com/bgsforums/index.php?showtopic=719546.

The news writer said, that Lorkhan would return, because we freed his aedric soul (in the Amulet of the Kings) and his daedric soul (in Sheogorath). Further we destroyed as Nerevarine his heart. The last point sounds realistic, but... Lorkhans soul in the Amulet of Kings? I never read anything about that. Do you know more or is this just a... lie?!

Akatosh and/or Lorkhan put Alessia's soul in the Amulet of Kings, and all subsequent Emperors followed the trend. This was the beginning of the Covenant/Dragonpact. I don't know how you could see that as Shezarr's soul being "trapped" in the jewel, unless you read really far into the wording of that Oblivion book that says Akatosh 'bound the bloody sinews of his heart,' or some such. Sheogorath has been described as the Sithis-Shaped hole that resulted from Lorkhan's dismemberment, but whoever wrote SI disregarded that entirely.

And disregard everything Nou Sepred posts, it is either gibberish or a joke no one gets.

Is the Colossal Black Soul Gem strong enough to replace the Amulet of Kings or is it a trinket by comparison?