i will always know you.

Harrow knows exactly how expensive it is to ensure that necromantic lineage. She knows that her parents miscarried repeatedly before she was conceived, and that ultimately two-hundred children died to make it happen. So maybe, someday, when she starts staring down the barrel of her own morality, and by extension the Ninth's mortality, she'll start thinking about that one baby that didn't die. Maybe she'll start wondering if, now that the baby's grown up, it has the capability to sire another baby that doesn't die - a baby that's actually Ninth. The technical capabilities are there. Even without vat wombs, Harrow was shown by her parents the necromancy for internal ova manipulation, and gamete recombination in general is a known quantity this universe, so making any heir is a real possibility at least. But to make a necromantic heir, that baby has to survive, and that baby has to be flooded with enormous amount of thanergey. The former is possible through Gideon, and the latter might be possible through Harrow, given the thanergetic overkill of her conception. She states that the babies alone generated enough thanergy to take out the planet, so is it possible that some of that raw power remains within her? It would be worth a try at a necromantic heir at least - if not by certainty, then by probability, so long as they make enough of them...which has got to be the most terrifying aspect for anyone for whom one Gideon is already too many. Once Harrow's parents die, and she closes off the Houses to external visitors, this possibility becomes an inevitability. There will be no other option but Gideon for the Ninth's bloodline to survive. Harrow doesn't realize this yet because she is a child, and at this point she's focused on preventing Gideon from escaping the Ninth. Crux, on the other hand, has already envisioned 14,000,605 alternate futures and the only one without Gideon becoming Harrow's completely unregulated consort and ruining the Ninth forever is the one where Gideon no longer exists. It's not just that Crux is disgusted with the premise that Gideon is not Ninth, not even House, and refuses to even pretend to assimilate to either. And it's not just that Crux's job is to protect Harrow and to keep her servants and prisoners and thralls bowed low beneath her, and Gideon threatens to rise above that pure and holy hierarchy just by virtue of her theoretical grotesque fertility - although I think that inversion of status is a huge part of Crux's impulses, too. Crux is one of the few that, knowing both of Harrow's conception and the truth her dead parents, perceives exactly how dangerous Gideon is to the Ninth. Harrow understands that Gideon is dangerous if she escapes, because she can reveal the Ninth is leaderless and unprotected. But Crux understands further that Gideon is also dangerous if she stays on the Ninth, because someday Harrow's stern resolve to hate her surly thrall will crumble beneath same temptation as her parents. And Harrow will choose, to her own utter humiliation, to maintain the Ninth's bloodline, at any cost. Gideon as consort will hold hostage both the Ninth's secret past and its only future, placing her in an enviable position to make significant demands about what the Ninth becomes. So because Gideon cannot leave the Ninth, and because Gideon cannot stay on the Ninth, from the moment the Ninth's doors shut, Crux knows that Gideon must die. This is the only way he can see to protect Harrow, both because he cares for her in his own horrible misguided way that also strips her of all agency, and because protecting Harrow is his actual job description. To Crux, Harrow is the Ninth, and he must keep her from realizing, someday, that Gideon is also the Ninth. Harrow alone cannot be trusted to be tempted otherwise. Crux is on the Ninth's front porch racking his shotgun wearing a shirt that says "10 Rules for Dating My Daughter", and every one of those rules is You Will Drink the Poison.