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were the DE around before the Horus Heresey or did the eldar divide afterwards?
The fall of the eldar race was before the heresy, in fact it was the birth of slaanesh that stopped all those warp storms which prevented the emperor from reconquering the galaxy at that time.
Essentially the fall of the eldar (and thus the creation of commorragh) initiatied the great crusade. The crusade didn't start immediately after it though.
Remember that it was the Fall that *created* one of the Chaos gods. I don't have a timeline handy, but I believe that the Fall pre-dated the Horus Heresy by a long way: centuries or millenia.
yes. the FGall occured millienia before the HORUS HERESY...