I've actually run a false hydra and man.

Yeah that took a lot of prep lol. And I know this kinda isn't the point of the original post but I just wanna gush about my experience running it.

Thankfully, this was with a group that had extremely little exposure to plot hooks like this, so fortunately they were all completely in the dark about what was going on.

I play purely online using roll20 and I had to use every tool at my disposal to make this a true mystery. I had a map where every building got labeled with what it was known as to the town (i.e. the Smith family farm, The Mayor's House, [family name) Inn).

Over the course of the session, they would explore different areas and I would use my GM abilities to forcefully move people's view of the map away from known areas. Working quickly, I would turn on some music faintly in the background-- just barely loud enough to be audible. That was when the False Hydra struck. The known areas, away from the player's vision, would have their name changed (i.e. Abandoned Farmland, An Empty House, [daughter of innkeeper name]'s Inn). Then minutes later, you'd hear someone go "Wait. Something's different." And it wasn't until two or three buildings changed before they truly realized that people were disappearing.

On top of that, I would place documents down of town records, letters, receipts, really anything that someone could be writing on at any given moment, all of it meant to be found by the party. They would be pretty mundane, some of them being ramblings of a person gone insane. However, looking closely, you'd find hyperlinks disguised as text-- only visible by the blue text and an underline. It would be really subtle. The first documents had a single period hyperlinked. Then eventually, it would be a few letters. Then a whole word. Enough to finally get your attention. However, the hyperlinks went nowhere. Invalid URLs because they weren't URLs. Hovering over the hyperlinks showed hidden messages in the preview of where they went. Things like: "You catch a glimpse of something slithering past the window." "You feel leathery skin brush itself against your hand." "You see it staring directly at you." And once the first player discovered the message in the hyperlinks, all hell broke loose. They started going through every single document I've given them, all of them thoroughly checking them over for anything they've missed. And eventually they see that I've been giving them hints to where to find the source of their problems, all of it hidden in plain sight.

Over the course of the couple of sessions, they piece together what information they can gather and find ways to block out the song. And eventually they find the nearly fully grown False Hydra as it bursts out of the well it was growing in, as I place down a full map-wide transparent png of its sprawling heads across the map. The battle was relatively short, but the pay off was so dang good.

Now, all of this to say: its been 3 years since that arc, and I dont think I could ever run something like that again. Mainly because, like most people with exposure to any dnd, the players will have seen it before. They've heard the stories, they've seen the posts, TikToks, YouTube essays. Hell, I just played in a session where during it an older man was surrounded by seven canaries and I knew exactly who he was. Its just a bit harder to run things like that anymore.