"Don't worry. You should be able to withstand the pain-- Always are mine...Every strand of hair, every drop of blood."
The Demon King, Lucifer took Yomi's soul hostage 1200 years ago. No one believes this when Reiga tells the Zweilts in Story 61 and when Kuroto says that Yomi died 1200 years ago but Kanata answers with "only her body died, her soul was taken in the moment of her death". Kanata also said Yomi would be brought back if Lucifer got called into the human world, which is what Suzaku is trying to do.