Before the advent of Islam, a Christian community sprung up in an oasis on the modern-day border of Saudi Arabia and Yemen, called Najrān. According to Ibn Ishaq's biography of the Prophet Muhammad, people there worshiped a tall palm tree until a Syrian Christian leveled it with a prayer.
This was in the late 5th century, and the community prospered for about 40 years. Bishops preached from the backs of camels, as if they were pulpits. A leader emerged among them, called Saint Arethas.
Najrān was then ruled by a Jewish king, called Dhū Nuwās, who decided to rid Najrān of Christians. He dug an enormous trench, set it on fire, and burned 20,000 to death. You can read about the massacre in The Quran, which promises "the great attainment" for the martyrs and a fitting punishment for Dhū Nuwās:
"Those who have tortured the believing men and believing women and then have not repented will have the punishment of Hell, and they will have the punishment of the Burning Fire."
Before long, word of the slaughter spread all the way to Rome, and a massive army descended on Najrān. Dhū Nuwās, who would be the last Jewish king of Najrān, drove his horse into the Red Sea, beating it viciously so it swam deeper and deeper, drowning them both.