Where would Chaos go...

[GhostMalone]

The mark of a drug addled night waking up with 4 guys numbers carved in his rump :P

 

Well at lease one number carved

 

Damn slanessh

[Hyaenidae]

Wow, been a while since i bumbled my way into the CSM forums. Lots of new faces here too, good stuff.  :)

 

Anyhow, I've been thinking about this, and going back to the OP's line of thought, of creating an alternate timeline where Chaos is not only in ascendance, but is crushing the Imperium in a rolling tide of Hell cometh home, why limit yourself to the standard 13th Black Crusade? Why not, instead, cut a line where one of the previous Black Crusades actually breaks the lines and succeeds? Or another well known point in 40k history, like the Badab Tyrannicide (a la Huron uniting with Chaos early, fully corrupting allied forces, alliances with the Word Bearers and Iron Warriors, or even Abaddon himself?) Or really go crazy, and use the Age of Apostasy as a jumping board straight into Terra's heart?

 

This would give you a solid starting point, and a following history to corrupt completely using known points.

[Emperors Immortals]

I've been fond of teh idea the "Ghost War" of Abaddons 7th BC would have been a good cover for the AL to have got a foothold, sowing discord and ultimately sabotaging the Imperials ability to respond. They watch helpless as hundreds of warbands converge on Sol from the darkness.

[Goreshed]

Ya the Apostay would be an awesome point to start seeing as next to the Slave Wars there is little written on it.