"But should these Dragonfires fail, and should no heir of our joined blood wear the Amulet of Kings, then shall the Empire descend into darkness..."

I find it interesting that it's exactly what happened after Martin Septim's death. While it still stands, the Empire is but a shadow of its former self.

I think the darkness here means the Oblivion Crisis.
If it doesn't, then perhaps that was a hidden prophecy.

Nothing new.

It was foreshadowed as far back Morrowind by our good friend Wulf. You could say the collapse started as far back as Arena.

Though that line in particular seems to be talking about the barrier between Nirn and Oblivion weakening. At least, that's how I understood it.

EDIT: Yay, Ninjas!

In Arena, Jagar Tharn let the Empire go to the dogs and wars happened everywhere. In Daggerfall, there were kingdoms threatening break away from it and the Emperor needed Numidium to keep them in line. In Morrowind, several NPC's say that its days are numbered including the avatar of Talos himself, partly because Uriel was sick and people suspected his children were doppelgangers.

Besides Wulf, Caius Casodes also says: "The days of the Empire are almost over. When the Emperor dies, nine hells're going to break loose."

Empires always rise and fall, none last forever.

Personally im curious to see what would become of Tamriel if the Empire dissolves. Devil's Advocate surely.

The Empire has always been shaky since Tiber Septim's death. Yes they had a lot of good Septim leaders, but there were also a lot of bad ones. The Septim Empire is a flawed concept and as Daggerfall showed us not all nations within the Empire were very happy with it, and of course Morrowind where it was used as a vehicle for cultural change. Ocato really was it's last hope and if what is rumoured about Uriel's sons are true probably would have made a better leader than any of them and being from the Summerset Isles likely was one of the few who really new what Danger the Thalmor presented, but he was assassinated.

I think it's only a matter of time before the Empire and the Thalmor-led Dominion dissolve (the latter I think of which we will have a part of and will come from within the Dominion as opposed to outside Invasion) and Tamriel mostly becomes Independent nations. Tamriel is truly becoming the Arena again.

Its fall was foreshadowed repeatedly, only to be forestalled repeatedly. In Morrowind, people were convinced it was on its last legs due to that succession controversy, and that the Emperor was about to die. That got ironed over. In Oblivion, the Septim dynasty ended. It managed to reorganize under a non-Dragonborn Emperor. 200 years later, in Skyrim, it's strongly implied on all sides that the peace with the Dominion is only temporary. But even then, I've not heard of anyone, aside from the Thalmor (who are clearly planning to go to war again), expressing their belief that the Empire is done for.

At this point it's up in the air as to what will happen next.