[S15] Wikipedia (Reliability: 2).
Leo was born to Eudokia Ingerina who was at the time mistress of Emperor Michael III and wife of his Caesar Basil. Which of the two men was his biological father is uncertain. Basil legally acknowledged Leo as his son but his later treatment of him might suggest that he regarded Leo as Michael's son.
In 867, Michael was assassinated by Basil who succeeded him as Emperor Basil I. As the second eldest son of the Emperor, Leo was associated on the throne in 870 and became the direct heir on the death of his older half-brother Constantine in 879. However, he and his father hated each other and Basil almost had Leo blinded as a teenager. On August 29, 886, Basil died in a hunting accident, though he claimed on his deathbed that there was an assassination attempt in which Leo was possibly involved.