As a giant hunk with limitless potential, without Warp Essence, a Primarch was nothing more than a genetically enhanced human with a powerful physique.
But there were no ifs in reality. Ever since the War in Heaven erupted, the Warp had been smashed into one enormous cesspit, and the once-stable Sea of Souls had spawned all manner of powerful Chaos monsters.
Over the past tens of millions of years, as the Chaos Gods rose to power, the laws of reality had been split apart and devoured one by one. The age when everyone was as mighty as a dragon never returned; only a few scattered fragments remained, lost in corners where no one noticed them.
The Four Ruinous Powers had collected the more powerful fragments and kept them as genuine treasures for a rainy day, until a hulking figure wreathed in golden light came along and traded them away.
Perhaps they had been too confident, or perhaps they had seen that the Emperor was essentially one of their kind. Either way, the Four Ruinous Powers had never imagined the Emperor would welch on the deal. Everything had been fine one moment, then he'd turned around and pretended he didn't know them.
We don't know the precise terms of the deal, but judging by what happened afterward, we can infer that the Emperor hadn't kept his promise. He hadn't wanted to hand over a single one of the twenty-one Primarchs. In the end, his attempt to finance the deal on credit had fallen through, and the Four Ruinous Powers had left him half-paralyzed.
Made from fragments of the Warp and the pinnacle of human biotechnology, every Primarch was a treasure of the Galaxy. Their endurance and fighting spirit were astounding, and they offered limitless possibilities.
Take the Gene-seed that sustained the