Timeline before Subaru's arrival:
400+ years ago: 6 witches roamed the world. They were calamity manifested, although they had no ill intent.
Ekidna, the Witch of Greed created the Sanctuary to shelter oppressed half-demihumans who had no where to go. She used the Sanctuary as a laboratory for various experiments including immortality. Ekidna creates Betty as her daughter spirit, and takes in Roswaal the First as her apprentice.
400 years ago: Satella became the 7th witch, and was driven mad. She killed all the other 6 witches and devasted the world.
Sage, Dragon, and Sword Saint defeated her but was unable to perish her existence. They sealed her.
The Dragon sheltered Ekidna's spirit in the Sanctuary, so she can continue to exist as a ghost. He did this to have a counter-measure in case Satella's seal is broken. Roswaal continues the research of immortality and devices magic to take over the body of his family line to exist forever, and vows to resurrect Ekidna. Betty becomes the guardian of the library, waiting for her mother's call to free her from her position.
100+ years ago: Emilia is born between a unnamed human and a elf. She is raised by her aunt. Emilia befriends Betelgeuse, who is a Witch Cult member but is kind and generous and harbors feelings towards Emilia's aunt.
However, Witch of Vainglory and Bishop of Greed attacks the elven village to take Emilia.
In the ensuing battle her aunt is killed, Betelgeuse is driven mad and Emilia's powers goes out of control and freezes the entire elven haven.
20~30 years ago: A great Demihuman war was fought between the human kingdoms and the demihuman kingdoms.
Wilhelm the "Sword Demon" makes a name for himself, and is hailed as a war hero. He falls in love with a girl named Teresia who is later revealed to be the current "Sword Saint". He manages to defeat her in duel, and proposes.
10 years ago: Teresia loses her power as "Sword Saint" in middle of the battle with the White Whale of Gluttony, and dies.
The power had transferred to the next generation of Sword Saint, Reinhardt, Wilhelm and Teresia's grandson. Wilhelm blames him for the death of his wife, and the two become distanced.
6 years ago: Emilia is released from her frozen seal and meets Puck. She contacts with Puck and vows to free her people from the icy seal. At an undetermined time later she joins the race for the Queen Candidacy.
4~6 years ago: The oni village where Rem and Ram lived is attacked by the Witch Cult. Everyone is slaughtered, and the twins are taken in by Roswaal.