Voodoo (ブードゥ,Būdo~u?) is an African diaspora religion, a mishmash of Afro-Caribbean witchcraft and necromancy.
Shinyaku Toaru Majutsu no Index
A member of the true Gremlin, Zombie, has a background rooted in Voodoo as well as actively drawing in a great number of cultures, giving her an affinity for almost anything which allowed her to devise a spell which would allow the Magic Gods to safely enter the world after their Hidden World was breached. Toyama Luca of the Kamisato Faction also makes use of Voodoo-based magic as a follower of Legba Atibon.
Uses of Voodoo and associated concepts in the story
Legba Atibon: A loa who serves as the intermediary between the loa and humanity, standing at a spiritual crossroads, and a facilitator of communication, speech and understanding, believed to speak all human languages.
Luca follows Legba Atibon and makes use of his role as a deity who manages ceremonies (indirectly appearing in all Voodoo ceremonies through his role as intermediary), therefore not just existing on a specific ceremonial day but at all times, to achieve control of subjective time, allowing her to change the age of her external appearance and prepare magic normally once-in-a-decade in seconds.
Sacrificial Rituals: There are some pacts associated with Voodoo which involve a person selling their soul to gain power and require them to regularly sacrifice others, otherwise they pay with their own life. These sacrifices can't just be anyone and have to be people whom it pains them to sacrifice, such as family members or lovers.
Salome refers to Voodoo practitioners who, having sold their soul, must annually sacrifice someone dear to them or forfeit their own life, when describing her own sacrificial magic and what she has sacrificed.
Zombie: Originating from Haitan culture and associated with Voodoo, a dead person reanimated as an undead or a person placed into a death-like trance, through magic, to serve as a slave.