The theory that Nanika [Alluka's "inner self"] is actually one of the five great calamities from the The Dark Continent. Although there isn't that much on it yet, I think there's enough to warrant the discussion.
Here's the key points
- Of the five its speculated to be the "Ai Gas Life form" for a few reasons. The first is that some similarities could be made from it and Nanika. Such as the black eyes and that Nanika often says "Ai"
- We saw how the "Ai Gas Life Form" leaves its victims in a secret laboratory and it looked suspiciously similar to how Nanika leave its victims. Mangled twisted remains of a person.
- Ging mentions at one point that victims of two of the calamities have appeared in the explored known world. One of these is the Gas Life Form. The other is Papu. This would make sense if Nanika is actually one of them as one of side effects of failing to meet Nanika's three demands is not only killing the person being asked but that persons closest family/friends. It does depend on the scale of the previous wish prior to the demands though. Basically, it makes sense that victims could be found around the world. It fits within Nanika's powers.
- Silva Zoldyck spoke about Alluka/Nanika and I quote:
"That is not a person. It is something that came from a very different place. Something dark" - At first it's easy to assume that all there is to this is Silva not seeing him as a son because he was very "messed up" even by their standards. However, what if he was actually being literal. As in the "place" he's talking about is literally The Dark Continent? Is it a coincidence that the place was revealed the very next arc?
- When Nanika's powers were surging, everyone was in shock. Even the Zodiacs were commenting on how they never felt anything quite like that. It was different
- One of the biggest questions is that how could this possibly happen... how could Nanika have seemly "possessed" Alluka? We know that the Zoldycks have travelled to the The Dark Continent. We saw a flashback that had Zzigg Zoldyck [Zeno's Father] with Netero in the midst of those dark lands. What if Zzigg unknowingly brought something back with him, what if the Ai possessed him? Perhaps when Zzigg died [assuming he did die] the Ai latched itself onto the closest living thing it could find, in this case Alluka as a small child.
Those are the main points, as for how I think this could fit into the story from the Zoldyck's perspective?
I think by the time the DC arc concludes, word of mouth about Alluka will have gotten around. It'll be known and revealed that Nanika is from the Dark Continent. At this point knowledge of the continent itself will be general knowledge so knowing that one of the creatures there is living in their own lands will cause havoc. People of all sorts are going to get involved. Hunters, assassins, mafia groups - some will want to control Nanika for power, others will want to kill it out of fear. A world-wide bounty will be put on Alluka's head and by extension, Killua's - especially if it gets out that he can control Nanika fully. This will make the Zoldycks overall but especially Killua and Illumi a lot more desperate. Things will get crazy.
I think a major plot-point in this will be the death of Alluka. I think that's the next major step for Killua's development. If you think about it he's the only one of the main four that has yet to experience any real loss. We've seen him annoyed and cold but we have never really seen him pushed over that edge like we have with the others. He's always been more composed in comparison which is why I think its especially interesting that we saw more of that shell chipping away during the election arc when Alluka was threatened. I think it'll happen and Killua is going to go on a war path.
I think some seeds regarding this have been planted already. For example, Gotoh, someone he really looked up to was murdered and because of this the other Zoldyck butlers are hiding Gotoh's death from him. I think Togashi is waiting to reveal this to Killua when he's at his lowest point mentally which will just send him even further down that rabbit hole. And of course, this arc will also be when one of the most anticipated fights finally happens: Killua vs Illumi.
Finally, I think "Nanika" has to die... unless his power has a major limitation in what he can grant, otherwise it's just a more accessible version of the Dragon Balls. A walking plot-device to fix everything when shit goes south and that ain't good story-telling, knowing that despite all the damage everything will be okay in the end because Alluka is OP.