What is year 666 ad?

In the year 666 AD, several events took place in different parts of the world. Here are few notable incidents:

1. The Byzantine Empire faced several defeats in Syria during the Muslim conquests. The Arab armies captured the city of Antioch and besieged the imperial capital of Constantinople.

2. In Europe, the kingdom of Essex in England was established, and King Sisebut of the Visigoths in Spain died, leading to a power vacuum.

3. In Japan, the Silla and Tang dynasties of China gained control over parts of the country. The Japanese Emperor Tenji, founder of the Tenji lineage, ascended to the throne.

4. The concept of zero was first introduced in the Indian subcontinent, and it was later adopted by Arab mathematicians, making it an essential part of modern mathematics.

5. The Anglo-Saxon Chronicle records that a "red crucifix" appeared in the sky during this year, interpreted by some as a sign of impending doomsday.

Overall, the year 666 AD was marked by political and military upheavals, religious growth and philosophical advancements across different parts of the world.