All you have to work with is Eldar myth, concerning the war in heaven and so on - because that's the only fluff which survives from the period when the Old Ones made war with the C'tan.
If you read the Harlequin fluff, and, IIRC, Codex: Craftworld, there are descriptions of the Harlequin dances which show the Laughing god tricking the C'tan into devouring each other through illusion alone (The void dragon, I think?) and eventually driving them insane.
I would say that he has intimate knowledge of the C'tan - the Webway (of which he alone knows every nook and cranny) can take him almost anywhere in the Galaxy. His agents, the Harlequins are similarly mobile, and have access to all the knowledge and assets of the Dark Eldar, Craftworld Eldar, and most importantly they have access to all the knowledge in the black library, which would contain everything that the pre-fall Eldar knew about the Necrontyr, C'tan and all other things.