Ceremony of Ba Moun

The Ceremony of Ba Moun (バ・ムーンの儀式) is a Voodoo ceremony which involves a contract with a demon. Described as the worst and most wicked spell in Voodoo, the demon grants the caster's wishes, but in return they must annually sacrifice someone they care for, otherwise they will be devoured by the demon.

Background
It isn't known when and how the ceremony was devised, though it has apparently been used enough times for it to become well-known and infamous.

Toaru Majutsu no Index SS: Agnese's Magic Side Work Experience
While Agnese Sanctis, Lucia and Angelene were investigating the forensic cleaning at Nihili Padpois's restaurant in Barcelona, and considering the fact that he had supposedly lost his wife to a curse in Haiti, Lucia referred to the Ceremony of Ba Moun (though not by name) as the most horrific of the many curses in Voodoo, with the nuns briefly worrying about his daughter Solene Padpois, before identifying the true culprit.

The incident which the nuns later investigated in Frankfurt was the result of attempts by the magician Frau Halveria to escape from the curse of the Ceremony of Ba Moun, having previously researched the spell and failed to control it. Threatened by the curse, she made use of a delivery app to repeatedly arrange for young bike delivery workers to be sacrificed to the demon so she could temporarily escape from it (as well as hoping that someone else might be able to deal with it),  with the workers receiving a package and on delivering it to an abandoned building, being devoured by the demon emerging from the package, leaving only a bike and a pool of blood behind.

Having heard about the disappearances from Isabella Theism and discovered Angelene's secret side-job, doing bike deliveries for snack funds, Agnese Sanctis and Lucia had her participate in a sting operation. Posing as a restaurant manager, Frau Halveria gave Angelene a spherical backpack and remained at the restaurant as Angelene was transporting the deadly cargo. Angelene cycled to the warehouse designated as the delivery point, with Agnese and Lucia secretly following her on rooftops. As Angelene arrived at the destination, the nuns thought over the circumstances for things they might have missed and realized that the backpack contained the murder device, but their warning to Angelene was too late as the demon emerged and set its sights on her.

Pedalling for her life, Angelene fled from the pursuing demon and on her comrade's advice, managed to retrace her trail and break into the restaurant, to Frau Halveria's horror. With the demon locked onto both of them, Frau Halveria fled with Angelene to the kitchen, forming barriers to slow their pursuer down, while explaining her circumstances. The demon soon broke through the barriers and the doors, as Frau Halveria mixed a drug to induce a state of pseudo-death in a final gamble, intending to give it to Angelene and sacrifice herself, having had a change of heart after listening to Angelene's words. However, Isabella (having been prompted by Agnese and Lucia) was able to deal with the demon through use of life-like fake corpses modelled after its target, leading it to tear itself apart.

Salome Arc
During her encounter with Accelerator, while talking about how she had sacrificed her body through Internal Offering and how what matters for a sacrifice is how important it is to the one offering it, Salome referred to Voodoo rituals (though not referring to the Ceremony of Ba Moun by name) where one sells their soul to the devil and has to offer up someone whom it pains them to sacrifice once a year, paying with their own life once they run out of people.