So,I was replaying the tutorial and after you choose your birthsign Uriel says this : "My dreams grant me no opinions of success. Their compass ventures not beyond the doors of death. But in your face, I behold the sun's companion. The dawn of Akatosh's bright glory may banish the coming darkness. With such hope, and with the promise of your aid, my heart must be satisfied."

What does he mean by that?Does he foresee Akatosh becoming a major force which could destroy Dagon,or is it just the emperor talking rubbish?

Uriel Septim VII has insight that rivals the scrolls themselves. However in this context he is only refering to the Oblivion crisis and it's end-game. He's never spoke "rubbish" but he can at times be abstract and confusing. Such are the pains of royalty I suppose, especially when the royalty in question walked head-long to his own demise for the good of his empire, stopped a potential long lost deity from enslaving the world, and managed to keep the Illiac Bay from going into full-fledged war.

No matter the era, long live Uriel Septim VII. He will be missed.

I'd agree that in this case, he is probably refering only to the Oblivion Crisis. "Akatosh's bright glory" sounds like it could refer to what happens at the end of Oblivion's main quest. Whatever the case "The dawn of Akatosh's bright glory may banish the coming darkness." doesn't really give me the impression of destroying the world.

That's a really good point, Helseth may have manipulated the PC into killing Almalexia for him, but it was Uriel Septim who put Helseth into that position. I wouldn't be surprised if the "minister" who warned him the Nerevarine might be a threat had connections to the Blades.

Let's break down the metaphor, if things are still confusing for you: You are the sun's companion (who/whatever the sun is, you are it's companion). The sun is the rising of Akatosh's bright glory come to banish the coming darkness. Martin becomes Akatosh, so you can pretty much equate the two here. Therefore, he's saying, you are Martin's accomplice, who will turn into a dragon and close shut the jaws of Oblivion.