So how come when Alduin comes to skyrim reviving dragons and eating souls or whatever he does no one in Skyrim says: "oh no! its the world eater the worlds ending!". No one except the greybeards, other dragons, and the blades know who he is! Shouldn't people know that the guy who ends the world is here???

Dragons have been gone for centuries, all people really know now is that dragons are returning and attacking towns and villages. The greybeards and blades only know about Alduin because of their long history with dragons and Alduin. The only thing that doesn't make sense is the conversation with guards about how you defeated him and saved the world. People shouldn't know that seeing as nobody (alive) was there when you killed him.

Tbh, there's alot that normal people know about you that they shouldn't.