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Alluka is considered to have no soul or emotions by the Zoldyck family with the exception of Killua, and will grant one wish after a person fulfills three of Alluka's requests. After the requests have been completed, Alluka's eyes and mouth darken to black and Alluka transforms into a dark continent creature named Nanika. The wish doesn't have to come from the one who fulfilled Alluka's requests, as even an extra party can have their wish granted for as long as it was declared in Nanika's presence, who would usually state "Aye" as confirmation that the request made was granted. According to Killua, the degree of wish that Nanika can grant is probably infinite, going from briefly levitating a person into the air, to affecting probability to make a certain effect occur. However, the greater the wish made to Alluka is, the greater the next three requests will become. The one that must bear the cost of the next three requests is not the one Alluka granted the wish to; it is the next person who must do it. And if that next person declines to fulfill Alluka's requests four times, depending on the difficulty level of the previous wish, at least two people, he and his most loved one, will die. The other people who die, along with the recipient of the requests and the recipient's most loved, are in order of who the recipient spent the most time with regardless of whether or not the recipient loved that person. One example is when Kasuga, a servant of the Zoldyck family, declined Alluka's four requests, the cost of the billionaire wish made previously by Yasuha was transferred to her, and thus she, her lover, and 65 other people were crushed to death.

There are five rules about Nanika's power known by the Zoldyck family as a whole, but Killua is aware of some others by himself. Below are the rules known by the family:

If someone fails to fulfill Alluka's requests and gets killed, the difficulty level of Alluka's requests returns to level 1. That is the best time to make a wish.

If Alluka is making requests from a particular person, they cannot be moved over to another person mid-way. Therefore if that particular person disappears such as concealing themselves, Alluka is unable to make requests to anybody else.

If the person fulfilling requests dies mid-way, it will count as a failure and at least one more person will die.

Alluka cannot make requests of someone without knowing the person's name.

The same person cannot ask Alluka for consecutive wishes, although Killua seems to know of a way to bypass this condition.

Rules known only by Killua:

While Nanika can grant only 1 wish at a time, one can give Nanika several conditions to a wish, therefore greatly expanding the capacities of what one can ask of Nanika before granting another 3 requests.

If a wish is made for healing, Nanika must make physical contact with the target; there isn't any sort of cruel backlash as well. This may simply be a rule Killua made up due to his special privilege.

Killua, due to his relationship with Alluka and Nanika, is given special privileges. When he "commands" Alluka, Alluka's request will bypass any existing backlash and will be granted without any repercussions. If somebody else wishes other than Killua, they will suffer the same repercussions of the previous wish that was supposed to be given. This privilege has been deduced by Illumi, adding to his craving to control Killua in order to control Alluka's abilities.