Never underestimate the power of bro-hood, ~degeso.
Ortho siblings - Brother (Aizawa Masaki) and Sister (Ohmoto Makiko). But not before a very interesting exchange between Killua and Ikalgo, in which Killua offers Ikalgo the chance to be spared if he'll spill the beans about Flutter and who he's working for. Ikalgo reveals a sort of honor here - and he's not the first Chimera Ant to do so - by refusing to sell out. He also reveals an intense dislike of being called an octopus, and an aspiration to have become a squid instead. In the end he refuses to betray his comrade, electing to drop to his death at the hands (well, teeth) of the blood-crazed chimera-sharks below. But Killua spares him - because he "seems like a cool guy", and "in another life we could have been friends". The irony here is that for all his assassin's detachment and dispassion towards killing, it's an act of mercy that ends up saving Killua's life. Leave it to Togashi to turn a hipster chameleon and an octopus with tentacle-envy into standouts, but so he has here - as absurd as Meleoron and Ikalgo are (their very absurdity makes them stand out all the more in this grim and bloody arc) they're fascinating additions to the cast.
turns the boy into a human dart board - a board which the brother attacks by firing Nen spearfish at him which have no physical substance until they enter his body. As Nen abilities go this one is a bitch - it's nasty, and damn hard to counteract. As usual no detail is too obscure for Togashi, and he exhaustively explains how Ortho-san inflicts damage to Killua using the rules of Double-in/Double-out 501 (which anyone who's spent any time in pubs probably knew already). So does Killua - he mastered the game when he was six (or was it seven?) as part of his training, and as such he's prepared for the final attack because he knows where it's coming - a double-20, right in the middle of his forehead. It's only Killua's astonishing skill and versatility (he uses his electrical ability to send an impulse directly to his arm to grab the dart as soon as it touches his skin) that allows him to survive and only pretend to be dead, thus tricking the siblings into lowering their guard and losing their heads (but not before bragging to Flutter that they'd eliminated the target).
The blood is red, and there's a lot of it. Killua's last thoughts are of the idea this encounter has given him to get stronger is his fighting (foreshadowing check), and naturally of Gon, and how he wishes he could have been more help to him. It's here that Killua's act of kindness (some would say an uncharacteristic one, though I wouldn't) towards Ikalgo saves the day, as the octopus decides to betray his allies and save the boy who saved him. It's been tipped off for a while that he'd end up fighting in the Hunter camp, and this just about confirms it.
conversation with Meleoron (who might just be my favorite chimera ant). This scene is short but impactful, and pure Gon. Meleoron asks Gon why he trusted him, and the boy answers matter-of-factly (as he always does) that it was because Meleoron swore he was telling the truth. Meleoron's reaction is a quite literal eye-roll, as he lectures Gon on the dubious nature of promises and questions his putting his fate in the hands (foreshadowing check again) of such a naive kid - but it's Gon's response that matters. With a smile that's not at all beatific he tells Meleoron that if in fact he's lying it'd actually be easier, because then "I won't have to show you any mercy - I'll be free to destroy you."