Maybe the main recipe of the Dark Curse consists of water from all five rivers of the Underworld?
Not that I imagine this was their plan for the origin of the Dark Curse from the very start of the show…but this could actually work. It had a drop of True Love potion on it so that it could be broken, so what’s to say that there’s not a drop from each of the rivers on there too, and each contributes to the Curse in some way? Styx – River of hatred. (Someone choc full of hate and anger was needed to cast the Curse.) Acheron – River of pain, which Charon ferries people across to get them into the Underworld. (Giving the Curse the ability to transport between realms.) Lethe – River of forgetfulness. (Amnesia is totally a thing the Curse was about.) Phlegethon – River of fire, that leads to Tartarus. (The caster of the Curse had to toss the heart into a fire.) Cocytus – River of wailing. (The fandom.
OK seriously, I dunno, maybe the emptiness that all the people got caught up in the Curse had to suffer?) Bonus drop from Oceanus – River that encircled the world. (AKA the Town Line that kept everyone trapped.) Go around to each river with an eye dropper, drip it all on there together and badda bing, badda boom, you’ve got yourself a Dark Curse.
This post is my new favorite thing 🙂 One thought — it needs not be actual water from the rivers, it could be an attempt to replicate the magic of each river through other means. So in this sense, the Dark Curse might actually be the Underworld on a small scale. So perhaps it’s not that anyone has cast the DC in the Underworld, but that the UW is, in its essence, the same thing as the DC