In the Anime: What the hell Chrono? Why don't you use the horns you got back from Joshua? Why are you THROWING THEM AT PEOPLE? Then you would have been able to do what Rosette asked towards the end!!
Aion has demonstrated that he removed his horns. The question is brushed upon in the manga, but who is his contractor in the anime?
When Pandemonium tries to change bodies in the end of the Manga, Aion laughs and says that she has already destoryed any body she could have taken over, the page jumps to a cut of the contaminated demons. My question is, if Pandemonium could have taken over other demon bodies, why did they need to kidnap humans in the first place?
Given the insect hive-like nature of demons, all the non-queen demons are probably infertile. Any demon could be a temporary core as a stop-gap measure but to truly perpetuate their population they need a fertile female to be Pandemonium's "queen." If they could have used a human to breed a true queen by now they would have, meaning there are probably controls in the system that prevent a new queen from being birthed until certain conditions are met (like having enough demons to need and support a new ship/colony, which is the main reason you'd want to birth a new queen for a potentially immortal race).
Okay, so it is revealed in the manga that Aion already beheaded Pandemonium 50 years before the start of the manga. So, how is it that Pandemonium is still intact in the final issue?
I have thought about that too, since he even says she is the one who gave birth to him and Chrono, maybe she somehow managed to collect the pieces of her body and make grow or melt it somehow into a new one.
Aion made an arrogant mistake and brought the head back to the ship, perhaps? He could have lots of reasons for it, too - like using the information in it to program and command a new subservient Pandemonium that he could control, for demons' perpetuation - but the legion cells' regenerative abilities would make it a good opportunity for Pandemonium to reassemble.
What Aion actually cut the head off of 50 years ago was a "terminal" of Pandemonium, not the actual Queen herself. They say it a couple of times throughout the series. You can actually see another terminal floating behind Joshua and Rosette in volume 8, when they're fighting each other.