So I have finally made a reference sheet for my version of the 11th Primarch, Siveris. The god in the machine. He landed on a planet that was fully run and controlled by AI and had been disconnected from the humans after the age of strife. When his pod crashed on the planet the AI of the world decided To experiment on the 11th to see what unholy creation they could make with a fresh human infant.... And that's how we got Siveris. It is unclear if there is anything organic or original left of the 11th Primarch. It is also unclear if the being known as Siveris is the 11th or something else. All that is known about Siveris is that his abilities are more akin to looking like magic with how advanced the technology is. An old record that was found is mostly blacked out but had one piece of information saying "not human, god in machine, kill on site."