"So, you finally did make it. I've been waiting for you, you throne-bound, angry, incre-he-hedibly hypocritical man. I'm going to feel right at home."The Star Child is a mysterious entity who lives in the warp. He is a being of immense power that is connected to the Emperor of Mankind.
Official Canon Edit
The Star Child was a theory put forward as far back as 1st Edition WH40K. The theory is that if the Emperor were to die, there is a chance that he will simply regenerate in the warp and be reborn, much like his Shaman ancestors had. Because much of his soul is fragmented across the warp and yet tied down to a dying physical body, being kept on the Golden Throne might be preventing his soul from healing. However, this is not a definitive conclusion.
TTS Canon Edit
The Star Child, alongside Saint Celestine, are brought by the soul of Dominique to where Fyodor Karamazov and his remaining forces were found. The Star Child thanked Dominique for his work in preparing Karamazov, and set his plan into motion. The Star Child entered Karamazov’s body, filling him with divine power and transforming him into an image of the Emperor himself. Taken aback, Leman Russ recognized the transformed being as his father. Seeing as how the Star Child’s personality is much gentler than that of the Throne-Emperor, it is possible that he is the fragment containing the Emperor’s missing compassion.
- In the Q&A session following the episode premiere, Alfabusa and Randolph explained that the Emperor deliberately used Karamazov as his soul's vessel. By taking the most stubborn man in the Imperium, convincing him that he is the Emperor, and sending him into the Warp, the nature of the Warp would naturally cause him to actually become the Emperor by guiding the Star Child to him.