In The Beginning There Was Man
In The Beginning There Was Man, or ITB, was a massive user made questline that detailed the journey of the Emprah, further along the line. In this time, Chaos, Orks, Tyranids, and Necrons are all assailing the Imperium, which has begun to crumble. Many Space Marine chapters are lost completely to this. The entire journey can be found here:
The majority of the written adventure is done by one person, and unfortunately, has been discontinued. The thread is still, however, full of Awesome and Win. It is decently fleshed out, and while not All Guardsman Party big, still pretty large.