What do you think was the greatest threat to Tamriel out of the five main games?

I haven't played the first two and I never finished the main story of Morrowind (But I have played it extensively) so if they have different names please tell me and I'll fix the name.

Definitely Mehrunes Dagon invading.

Alduin returning as the World Eater is a prophecy and a role he abandoned because he wanted to enslave Tamriel/Nirn, not destroy it. Him returning did not signal the end times, but more so the return to the reign of Dragons over the world.

Mehrunes Dagon could have potentially destroyed not only Tamriel, but Nirn and Mundus as well had he been successful.

The Arena crisis posed the threat of an evil overlord, but not all-out destruction. Tharn was just a generic power-hungry-wizard-guy.

Dagerfall was mainly abou political manoeuvring. There wasn't even a real threat to Tamriel (unless the Agent did something stupid like try to use the Numidium himself...)

Dagoth-Ur wasn't interested in spreading his reach beyond Morrowind, he just wanted the Outlanders gone.

I voted for oblivion because it would bring about the end of the world as we know it (I know Alduin would have done that two, the difference is that when he does it, a new world is born)

The Daggerfall crisis is called the Warp of the West