" The Falmer discovere a magic that could make the sun stoping of appearing at all. So with time the Falmer would be conquering Skyrim"
The Thalmor can do feel thing if we are talking about trying to get into cities full of spiders as well as spider web.
It would be much more plausible (and make for overall better play/story) if they just discovered something that would allow them to create "globes" of darkness hundreds of feet or yards across. An item powered by a soul gem with the strength of the gem determining the size of the area of darkness. So, the job for the PC would be to infiltrate the falmer held area, locate the gem, and either destroy it or just take it to banish the darkness.
