I know that the canon around the start of the 40K universe has changed from edition to edition of 40K but I am sure that at one point the history was retconed so that it was the Old Ones who brought about the creation of the chaos gods because the only thing that could defeat the C'tan and Necrons was the power of the warp?
Reading the most recent history this does not seem to be the case but I know that Games Workshop has form in changing its history to fit the current line of products.
Am I right in thinking that, at least in one timeline, the Old Ones enabled the creation of the chaos gods as a side effect of using the warp to fight the war in Heaven?