Headcanon's About The Greengrass Blood Curse:
We know it only seems to affect the women of the family, since the characters it canonically impacts are Astoria Greengrass and Cereus Greengrass, and Cereus inherited it from her Father who was unaffected. Presumably Daphne Greengrass is also impacted, although seemingly to a lesser extent, meaning there is some variation in how this chronic/terminal curse impacts people. Cereus' mother, Aquila Greengrass, is also dead although that's apparently unrelated.
I tried to see if the name meanings might have anything to do with it, but Greengrass is a surname relating to agriculture, so I wasn't sure that that had much to do with it. Astoria has two possible meanings, either coming from the Latin "astor" meaning hawk, or being derived from the Greek "Asteria" meaning stars. Considering her line of the Greengrass family uses a lot of Greek names such as Daphne and Hyperion, I'm more inclined to think it relates to the Greek. Cereus' family line is a little different, since her Father's name is Gareth, and her name is Latin, specifically meaning "wax, candle". This is where we get the word "ceramic" from, I believe, although Cereus is pronounced with a K sound.
Aside from that, Asteria and Daphne do actually have something in common. They were both chased down be a God, Asteria was chased by Zeus (Dios) and Daphne was chased by Apollo (Apollon). Asteria turned herself into the Island of Delos to escape Zeus (Dios) and this is also where Apollo (Apollon) was born, along with his twin sister Artemis. Daphne was turned into the laurel tree in order to escape Apollo (Apollon). I also think it's worth noting that Skorpios, the monster Astoria names her son after, was created by Artemis with the explicit purpose of killing Orion because he wouldn't leave ladies alone and he was hurting the wilderness. He was also placed among the stars to protect the Pleiades from Orion, who wanted to get with them and that's why Orion disappears from the night sky right as Scorpius appears. So, I do think the curse probably originated from a man being upset that a woman was saying no to him.
Something else I'll note, the surprising link to Sun gods, Light gods and Cereus' name literally meaning candle. I have to assume this means the curse involves some form of burning up and/or hiding from the Sun, kinda like vampires, since Daphne was literally running from Apollo (Apollon) and we know Astoria ends up isolated in Malfoy Manor, unable to leave.
It is also specifically a blood curse, so I imagine the curse probably targets the cardiovascular system first. Either through various blood diseases or heart conditions, maybe it's a blood curse because they have to consume blood, I don't know, they're already maybe hiding from the Sun. Of course, vampires do already exist in Harry Potter so maybe that's just a coincidence JKTERF didn't think of. Of course, if the blood curse involves burning up like I suspect, I could easily see it being that their blood literally starts to boil. It would certainly help explain the very violent death Astoria goes through that Scorpius has to witness. It would also help to explain Gareth's desperation to find a cure for his daughter, although he does it in a very backwards way and ends up killing himself so maybe he should have just let his daughter be disabled.
I did consider that perhaps the Greengrass blood curse involves turning into plants, especially since their name has an agricultual meaning, but I feel like they wouldn't explicitly call out Astoria as being extremely sick and dying if she was just changing form. So, I don't think this is some plant variant of the maledictus curse Nagini has. Although it would be sick if they turned out to be related. Don't know how it would work since I'm pretty sure Nagini is Indonesian, and the Greengrass's do not appear to be Indonesian, but sometimes family trees get interracial like that.
Something else worth calling out is that the curse explicitly gets worse over time, meaning the Greengrass girls are dealing with chronic illness and disabilities from the beginning of their life, just in very mild forms. Astoria dies while in her late thirties, and the worsening does appear to accelerate with age, but the curse also affects Cereus while she's very young, meaning it's definitely a constant and permanent thing. The fact that pregnancy exacerbates things for Astoria also means there's likely a link to their fertility and/or sexual anatomy. Which doesn't help the indication that maybe the curse came from a man who doesn't like being told no.
The link to pregnancy also makes me wonder if periods are worse for them, since periods are literally when the body is losing a lot of blood, and in severe cases people who experience periods can need to visit the hospital for blood transfusions every time they have one. It does also make me wonder if there's anything gastrointestinal going on, since the amount of nutrients being absorbed by a baby might course issues for someone whose digestive system doesn't work properly 100% of the time.
Something else I will bring up, the curse does also explicitly affect men. Not super obviously, but Gareth Greengrass carried the curse and unknowingly passed it onto his daughter, meaning that Scorpius Malfoy also definitely carries the curse. Scorpius is also shown very clearly to be much weaker physically than his peers, being very easily overpowered by Albus, who isn't really shown to be all that physically imposing in literally any other scene. It's not obvious, maybe it's a Scorpius specific thing (intersex headcanon is popular because of this), but it does seem like the curse still affects men in some way. This does also mean that if Scorpius had a daughter, she would be cursed with the same thing that killed his mother in front of him.
Also, something more theoretical that I don't think is canon, but I kind of want to throw out there, is the idea that perhaps the curse makes the Greengrass women tragically alluring. Like, they look like anyone else, but if the curse originates from a Greengrass woman saying no, it kinda makes sense that as punishment a lot of men would be lured in by the curse. And like, if you subscribe to the Malfoy's have Veela heritage headcanon, that would provide some interesting parallels between Astoria and Draco. Also, it would definitely explain why the entirety of the Cursed Child character list all have the hots for Scorpius Malfoy. Like seriously? Albus, Rose, Myrtle, Polly and Craig??? I know he's got autism rizz but it's definitely more than that. It just feels like something that kinda makes sense, but there's no real evidence for it. We don't know anything about Astoria's love life aside from marrying Draco, so it's hard to say for certain that she dealt with any sort of allure, especially since Daphne doesn't appear to have dealt with any of that, we'd know since she's in Harry's year. But IDK, the curse also just impacts her less in general. It would certainly assist in explaining why Astoria in particular is so hated by purebloods and why she's viewed as impure despite being a pureblood. Just an idea, doubt it's canon.
So, to summarise, I think the curse was inflicted upon the Greengrass women because a one of them said no to a man and he didn't like it. The curse involves various cardiovascular issues that they will deal with from birth, and get exponentially worse until they have no immune system, and their blood boils, eventually killing them in an excruciatingly painful manner. Men will carry the curse, possibly making them weaker although fanon may dictate that it has more to do with intersexuality and blurred sexual anatomy confusing the magic, and if they have a daughter she will be given the curse. The curse also, for whatever reason, is worse for some women than other's, possibly just meaning the curse is taking longer to kill Daphne than the rest of her family. There does not appear to be a cure, and we get no indication that the women affected by it even want one. The only mention of a cure comes from Gareth deciding he wants to cure his daughter, and failing spectacularly while also putting Harry Potter in the situation to make an Unbreakable Vow to help her, leaving her an orphan instead with Harry Potter's weird promise to fix her in the air. Yeah, adult Draco's disdain for adult Harry makes more and more sense the more I look into things.
But uh, I don't know, I think vampire Greengrass's would be cool as Hell, but this is definitely what's more accurate to canon. Gotta love Astoria, the only canonically disabled/chronically ill character from the main gen, and yet she gets glossed over and ignored or outright mischaracterised because people don't like Drastoria.
