"The Roman army first encountered the cult of Mithras in Persia (modern Iran) during the reign of the emperor Nero although its origins in India have been traced back to 1400 BC. One of the many mystery cults that the Romans introduced from the east, Mithraism first appealed to slaves and freedmen but with Mithras's title Invictus, the cult's emphasis on truth, honour and courage, and its demand for discipline soon led to Mithras becoming a god of soldiers and traders.
Various stories survive to account for Mithras's birth. Often he is depicted springing from the living rock or from a tree; at Housesteads on Hadrian's Wall, however, there was a tradition that he was born from the Cosmic Egg.
Mithras's early life was one of hardship and painful triumph. Finally,he captured the primaeval bull and, after dragging it back to his cave, killed the animal in order to release its life force for the benefit of humanity: from the bull's body grew useful plants and herbs, from its blood came the vine, and from its semen all useful animals." - The Museum of Antiquities Online
This bull slaying scene - known as a tauroctony is found in virtually all Mithrae.