Mortis has a few valuable pieces of lore hidden amongst its general fluff... greater story arch revelations from the whole Horus saga and the 40K universe at large... but... But!

If this were a standalone book, maybe a 2 star rating, but this proves the Siege of Terra, as a series, misses the point. Mortis is heavily infested with subplots about utterly insignificant characters and the details of most its events... a lot of it feels insignificant.

I don't care to remember which Heresy novel focused purely on a titan crew, but here again we are subjected to extraneous amounts of detail regarding titan crews and titan politics... which, this time around, would actually be good as a standalone novel delving into the deeper politics and machinations of the Mars cult, etc during that whole "final hours" scenario. But it feels like distraction filler here where you are constantly being deprived of any real meaty interactions with what you came here to read about; the Primarchs, Horus's final transformations, the Emperor's actions/responses, the greater impacts/influences of the Chaos powers and the true reawakening of the Warp.

Everything you want is still being hinted at and vagauely glossed over to constanly favour peon characters you wish would just die (or at least I did)... sweep them off the board to make way for the real players in this grand opera.

This one reminded me of how heavy that sense of saga fatigue felt by the time I reached the end of the Heresy books.