About the "dark clouds" in AC, that i was arguing with Ryushikaze some time ago, exist some things that suport what i was saying, but i forgot about them, and didn't said anything.
"Kadaj, Loz, and Yazoo are referred to as physical manifestations of Sephiroth's spirit and will three times in the English text, with the specific wording used in Japanese being something akin to "will bodies of Sephiroth's thoughts" on one of those occasions and "phantoms of Sephiroth" on another. Also, Kadaj is said to represent Sephiroth's cruelty, while Yazoo represents his allure and Loz represents his strength." That means that they ARE Sephiroth. And they can controll part of the Tainted Lifestream(TL), like when Kadaj corrupted the water in the Sleeping Forest. If they can manipulate the TL, that means that Sephiroth can do the same. They used the the TL to create the Shadow Creepers(SC) too, "because materia allows someone to tap into the Lifestream's energy and manifest it as a spell"(including Summons), and the SC are Summons(with capital "s"). But we never saw them in the game, meaning that they are created by TL.
Now lets see, the SHM that are "weak" parts of Sephiroth's will(compared with Sephiroth himself) can manipulate the TL and even create life with it(the SC)! I think that is not hard to understand that Sephiroth(much more powerful than the SHM) could use the TL to simply cover the sky. Even because he said that he wanted to use it to reshape the face of the planet, and after this, the "dark clouds" begun to descend to "attack" the planet.

If you are reading this Ryu, you are probably thinking "But the RF never directly said that the clouds are TL", and you are right. But i was(and i am) only trying to make you understand that sometimes, we don't need direct answers, because we have just to understand that 1+1=2.