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I noticed on warhammer 40k wiki (and all over the internet) it states that 2.3 trillion people died in the Horus Heresy, and this number becomes 4.6 trillion if one includes the Great Scouring. There's no citation I can find for this figure (which is why it isn't on lexicanum I imagine), but does anyone know where this figure comes from? The specific source would be appreciated. Thanks.

I'd imagine it's more

40k wiki is pretty bad about citing sources. It's entirely possible those numbers were just made up out of thin air at some point (e.g., someone threw those numbers out in a forum post somewhere, and it got repeated enough times that it entered people's headcanon).

The numbers were probably higher. Hive Worlds can have hundreds of billions or even more than a trillion inhabitants, and so those number would be in line with a handful of hive worlds being wiped out.

I'd imagine that the numbers will be increased by the time Forge World ad Black Library have finished with the Heresy, they've really been upping the numbers the of legion's sizes and the number of wars fought by the traitors in the seven years between Istvaan and Terra.

The 2.3 trillion value probably comes from before they started doing the Heresy in great detail, for example the old fluff stated that there were only a million Imperial Army troops on terra which for arguably the most important battle in the fluff is nothing.