"Don't worry. You should be able to withstand the pain-- Always remember...you are mine...Every strand of hair, every drop of blood."
The Demon King, Lucifer, is adored by many duras. Elegy states that it’s an honor to be bound to “that noble personage” then chastises Luka for leaving that position for a mere human.
By giving his blood to a duras, Lucifer forms a covenant with them. This gives him complete power over the duras. He formed such a contract with Luka. He placed his blood in Luka’s Brand Zess causing it to turn bright red. Pain is present during the process but Lucifer states that it won’t be unbearable. He later makes a contract with Luze to replace Luka who left Infernus to be with Yuzuki.
Long ago, the Gious sought great power. They researched summoning duras on the level of Demon Lord and successfully summoned Lucifer. However, he was too strong for them to control and he destroyed almost the entire province before the Giou could send him back to Infernus. Still, the Giou elders were determined to keep Lucifer’s power. They embedded a piece of Lucifer into one of the strongest young Giou men, Takashiro. The fragment ate at Takashiro and would have drove him into a mad frenzy. To prevent this, they used the Evil Eye to erase Takashiro’s memories and make him believe he was human. A parasitic duras was placed in him as well to keep Lucifer’s fragment under control.
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". Lucifer told Reiga that if he summoned Lucifer to the human world, he would return Yomi. However, once Lucifer enters the human world, humans will be destroyed. Summoning Lucifer requires three of the most powerful grimoires: the Key of Raziel, the Key of Solomon, and the Key of Enoch. Each book needs a necromancer to use it.