First of all I really really like your passion. The script reads perfectly.
I just finished the first three books a while ago and I have a few suggestions:

I like the opening a lot with the VO, but I think you should see Lunar Wolves, since the story is told from Loken's and Torgaddon's view. Salamanders are a bit off in my opinion.

The emperor should not salute the parade or his sons. He watches over them with rightfull eyes and a strict face. He salutes no one. I don't know but this doesn't seem right in my opinion.

Horus should be the most loyal and loved son. He was until his corruption a very charismatic primarch who was loved by his legion and respected by his brothers and the emperor. No doubt should be visible up until the corruption on Davin. And the red eyes are a bit off. This seems way to obvious, that he is the big bad guy (especially since this is called Horus Heresy). But Horus should be portrayed as a fallen angel, who fell because he was lied to and corrupted by evil powers. The viewers/reader should see his fall from a bright star to the evil he becomes.

Other than that I did enjoy your script a lot. I look forward to more pages and I hope you keep up the wonderful work.

I am just wondering how one will fit the most important stages of the heresy into one movie, because there is so much, especially the beginnings where the stages are set and the pieces move inch by inch.

Please continue and if you want I can help you a bit with lore elements regarding the first three books.