All Primarchs, all the time

In Ancient Egyptian mythology, the god Osiris was a king betrayed by his brother Seth. He was murdered, dismembered, and his body scattered across the land. His wife Isis gathered the pieces and restored him, but he could not return to the world of the living. Instead, he became a god of an underworld kingdom.

And Osiris wore a crown with ostrich feathers, yeah. But it's not just that!!! They are a symbol of his kingship, but they are also connected to the feather of Maat. In the Hall of Judgement, the heart of the diseased was weighted against the feather of Maat. If the heart is lighter, the soul passes into the afterlife, and if it isn't, the person is to be devoured by the soul-eater Ammit.

Maat means cosmic order, truth, justice, the balance to be maintained by the king. All those cool pyramids were built by the Ancient Egyptians to maintain Maat.