Old armor pillars and painting
In the mission “Servant of the Machine,” as you approach the temple, you can see multiple busts of armor in the arena. These busts show an old variant of Space Marine power armor called the Mark III Iron Armor, which comes from the Great Crusade period, which occurred during the 30th and 31st millennium. This occurred prior to the Horus Heresy, so it was a period where Space Marine legions mostly got along.
The details also match up with the paintings that adorn the walls of the Temple of Thassean, which depict Space Marines battling to unify the galaxy during the Great Crusade. The artwork is reminiscent of art from multiple books from the Horus Heresy series, including an artwork titled Abakhol by Sam Wood, featured in the Visions of Heresy book.
