A long long time ago, in a warp far far away...
Khorne was drunk off his ass when Slannesh walked into his door. In his intoxicated state Khorne though that Slannesh was a giant walking pool of blood, topped off with a big skull. Slannesh noticed the man-sized bottle of fermented souls and jumped on the opportunity to get her man. He/She/It seduced Khorne, and two weeks later, Slannesh found out he/she/it was pregnant.
Khorne was furious as Slannesh admitted to what he/she/it had done, and immediately took he/she/it to court. The matter of getting a restraining order was resolved easily, angering Slannesh. But the true blow was when Judge Tzeenetch delievered the sentence that because his Lords of Change couldn't figure out what exactly Slanessh was, the child would have to be given to Khorne.
This made Khorne happier than he had ever been, and for a moment war stopped all around the Matterium. But Tzeentch was true to his ever changing nature, and decided a happy Khorne wasn't a good Khorne. His lawyer, God Malal was furious, as was God Nurgle, slannesh's lawyer.
In the end, Judge Tzeentch decided neither parent was suitable for TEH EMPRAH and sent him to the orphanage known as... Terra.