League of Assassins
The League of Assassins is an ancient monastic order that stretches back to the end of the Hyborean Era. It was founded by spiritual entities that balked at the return of the New Gods to Earth, following Ragnarok and the resurrection of the Demiurge; their goal was to preserve the life force of the planet itself, and they saw humanity as a threat to that plan.
The League has been responsible for the downfall of several major civilizations and destroyed many cities. Constantinople, Rome, London-- many times over the centuries they have purged huge swathes of the population in order to 'reduce' the human footprint. They were directly responsible for the Black Plague being unleashed in Europe.
Their leader is the immortal known as Ra's Al-Ghul, a child of Arabia nomads born in the 14th century CE.