I have been reading through the Horus Heresy, and after the attack on Prospero we learn that Magnus was shattered into shards. I know that other Black Library books set in the 41st millennium+ have also explored these shards of Magnus, but my question is:
When did the Black Library/Games Workshop source material first mention that Magnus was shattered into Shards? Was it an idea that was first shown in the Horus Heresy series, or had it been shown earlier then it appeared in the heresy books? Has it always been part of the story of Magnus?