The White Rabbit is an experimental drug created by Promestein. It contains the pure dark genes of Alice I (First), manifestation of darkness itself; the White Rabbit's purpose is to mutate one into the ultimate dark being.
However, not just anyone can take the White Rabbit; the person's genes must be closely related to Alice I's. Only Promestein, who imbued her body with monstrous seaweed which is the closest link to Alice I, and Black Alice, a direct descendant of Alice I herself, are known creatures to have been successfully mutated by the White Rabbit. It is presumed that Goddess Ilias's body, the manifestation of light itself who wished to gain the dark genes, would reject the White Rabbit as she is a being of pure holiness.
The drug is first mentioned after Luka and Alice push Promestein back at her lab under Remina. The scientist then tries to inject the White Rabbit, despite the drug's incomplete stage, but Black Alice stops her, saying it is too early to use it. After Promestein is teleported to the Drain Lab, she informs her Seekers of Truth that Black Alice is aware of the White Rabbit, but they should be fine as long as the drug continues developing. Meanwhile, Ilias questions what Black Alice did with the White Rabbit, and mentions that being a god is not pleasant as it seems.
When Luka makes his way to Heaven, Black Alice and Promestein initiate the plan to betray Ilias; Black Alice injects the White Rabbit in her and swallows Ilias, becoming the ultimate entity of light and darkness. Promestein then leaves to confront Luka and also injects a White Rabbit of her own, planning to swallow Luka to also become the ultimate entity. However, Luka defeats both Promestein and Black Alice despite their mutations, but Ilias emerges from Black Alice's remains and reveals she staged the creation of the White Rabbit and allowed Black Alice to swallow her in order to become the ultimate entity herself.