The Burning Era (1000 - ~700 years ago)
The mortal kingdoms of Terrenia were in chaos. Demons began possessing people, stealing their bodies and causing large scale horrors wherever they went. At first, demons targeted those with great magical power as it seemed as though the demons wanted only to cause as much damage and death as possible. As those who could wield magic died out, demons began possessing regular people as well. Anyone could be a demon and the mortals races were nearly wiped off the face of Terrenia.
The elves retreated to their woods, killing anyone who would enter their land in case they might be possessed. The dwarves sealed their mines and cut off all communication with the outside world. The Fae disappeared and were never heard from again.
As the population dwindled, a faction known as the Templar began to fight back against the demons and save humanity.
The Best Last Bastion (700 years ago)
Near the end of the Burning Era, the city of Tena was founded. The founding of the city as according to the Church of Light, was that the Goddess, Talena, came to the city to save its people from the demon invasion. She taught the Templar how to destroy a demon which kept the city of Tena safe, giving it the nickname the Best Last Bastion.
The Templar began a long, multi-year campaign to travel to every corner of Terrenia, ridding the land of demons and teaching common folk methods to identify, isolate, and kill demons. The end of the Burning Era led way to the Holy Era.