The White Rabbit is an experimental mutagenic drug created by Promestein and is a key element in Chapter 3. It contains the pure dark genes of Alice I (First), the 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 ancient monster 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, as the manifestation of light itself would reject the White Rabbit due to the incompatibility of light and darkness.
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, and then teleports Promestein away to her Drain Lab. There, 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 asks Black Alice what she did with the White Rabbit, and mentions that being a god is not as pleasant as it seems.
Nearing the climax of the war, Promestein and Black Alice are revealed to be in cahoots all along. Black Alice had combined the White Rabbit into her body, its power laying dormant inside her. Promestein intends to run a biopsy to finalize the White Rabbit by the next day.
When Luka makes his way to Heaven Black Alice and Promestein initiate their plan to betray Ilias; Black Alice injects the completed White Rabbit in herself and swallows Ilias, becoming the ultimate entity of light and darkness. Promestein then leaves to confront Luka and also injects a different formulation of White Rabbit optimized for her that Black Alice is unaware of, planning to swallow Luka to also become the ultimate entity while implying that she will use that power to betray Black Alice. 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.