Officially Gods, Demi-Gods and Heroes was the last supplement to the original D&D rules, but wait! There is another. I understand why Gygax did not want to make this a supplement, however, as it is basically a substitution of the CHAINMAIL rules on which D&D is based, so this was already around, it just wasn’t called D&D. 

Let me explain: D&D started out as a short rules addition to a miniature war game called CHAINMAIL, more similar to today’s Warhammer than D&D. As CHAINMAIL was focused on historical battles, Gygax had the idea of making a Fantasy supplement for it, and that’s where D&D comes from. Now, in 1976, D&D is bigger than CHAINMAIL and it makes little sense to presume that everyone will know the miniature game rules. So Gygax makes this extra manual to completely uncouple D&D from CHAINMAIL. It isn’t particularly useful, though, because most players by this time were already using some kind of “theatre of the mind” for the most part and therefore these ultra-detailed positioning rules would not stick, with some exceptions (like flanking, distance penalties, etc.).

Another reason that makes these rules kind of jarring with how people were actually playing D&D is that they are mainly applicable to large armies fighting each other, which very few people were doing in their D&D campaigns at home. So, you look at the diagrams on this manual and the large battalions of figures are kind of unrecognizable as D&D, again feeling more like the kind of thing you’d get in Warhammer or other war games. It’s still an interesting manual if you want to know how much D&D actually changed from the early days.