"At some point in the Heresy, Fabius did something unprecedented outside of the most twisted fan-fiction. He created a "daughter", from his own gene code, which he named Melusine. At the time, she was the pinnacle of his work on creating life but something in her making went horribly wrong and Fulgrim ordered her to be cast into the warp out of fear of what she might become. Let that sink in for a bit. Melusine then went on to wander the realm of Slaanesh, where she became demonic in the process. She would occasionally give vague warnings to Fabius in the future, holding some loyalty to her "father"."
This section is incorrect, this took place on Harmony, so after Fabius reunited the legion after Skalathrax, but before Abbadon threw a ship at him.
Also note that he would be dead by now, but GW didn't let their author kill him off even if the story (Black Tide) was totally nullified by this decision.