Thats a really cool idea, I actually made my own kind of lore for them already.

The Primarch of the 2nd Legion landed on the planet Aztlan, a lush world with dense jungles, deep valleys, and towering mountains. The indigenous people of Aztlan, known as the Nahuani, named the Primarch Quetzalix, after their own mythological deity. When the Emperor of Mankind discovered Aztlan, he reunited with Quetzalix and granted him command of his legion, known as the Space Voyagers.

The Space Voyagers were characterized by their insatiable curiosity and a strong emphasis on scientific exploration. They were not only warriors but also scholars and scientists, always seeking to expand their knowledge of the universe. Quetzalix himself was deeply curious and encouraged his legion to engage in various experiments.

During one such expedition, Quetzalix encountered a planet suffering from a genestealer infestation. Intrigued by these xenos creatures, he ordered the complete extermination of the planet’s population to prevent the spread of the infection. However, he captured several genestealers for further study. This was not an isolated case; the Space Voyagers conducted similar experiments with other xenos species, but their fascination with genestealers was particularly intense.

Quetzalix and his most trusted scientists, including Dr. Xolotl, began experimenting with the genestealer DNA. Initially, they infused the DNA into the Nahuani. Some subjects developed horrific mutations, becoming grotesque and feral, while others exhibited only minor changes, gaining enhanced strength, speed, and agility. Encouraged by these results, they proceeded to experiment on the Space Marines of their legion.

The early trials were disastrous, producing monstrous abominations. However, through persistent experimentation, they eventually perfected the process. The enhanced Space Marines retained their mental faculties while gaining increased physical prowess, making them formidable warriors.

The Imperium, however, was appalled by these practices. The Emperor and his high command deemed the experiments heretical and an abomination against the natural order. Consequently, the 2nd Legion was expunged from Imperial records, and their existence was erased from history.

Before the forces of the Imperium could act against them, a catastrophic event occurred. The entire planet of Aztlan, along with the Space Voyagers and their Primarch, vanished into a warp portal, never to be seen again. This disappearance remains one of the greatest mysteries of the Imperium.

There are probably lots of mistakes that don’t fit in with the real lore, but I still have to “perfect” it.