Planar Defenders

Purpose
The group was organized during the Era of Elves to pool all the knowledge and skills related to planar travel so as to prevent something from coming to the Prime Material Plane and invading. The group regulated and controlled most of the planar magic and magic items, but unfortunately they were naive and became exposed to dark forces beyond this world, leading to their downfall and the Era of the Fiends.

Role in Society
They were chroniclers, librarians, and magical scholars. They knew all the knowledge of the planes, the creatures that inhabit them, and how to travel the plains, at least all the knowledge known to mortals then. They would always seek out new knowledge.

How They Ended
A few members were exploring the Nether Planes too closely, not knowing how the planes worked exactly, and they became corrupted. Those corrupt agents allowed several high ranking demons and devils sneak into their world. After arriving, these agents helped them plan an invasion which ultimately led to the destruction of their group and the beginning of the Era of the Fiends.