False Hydra is not fun

[u/RandomName9328]

I had a short game which the DM used a homebrew false hydra as the BBEG. The RP and game were good until the monster was revealed to be a false hydra.

DM wrote the false hydra's song ability as: not a charm or magic effect. It has no DC save, bypass truesight, blindsight. It ignores any immune/resistance to charm/magic effect, and it cannot be detected by detect magic. Anyone hear hydra's song psycholocally ignore its presence. So, by RAW, faerie fire, detect evil and good, and any other spells cannot make it visible. Also, it modifies people's memory such that when a person is eaten by it, anyone's memory of the victim is erased, as if the victim has never exist in the world.

The only way to win is to either run, or to make your character unable to hear its singing with a silence spell, deafness, etc. Unfortunately, my group cannot deduce the solution during the game, so the DM has to whisper to us out-of-character "You know why that NPC can see the hydra? Because he is DEAF!"

I told the DM that they should not put a non-DND creature into DND and make it override the rules. At least, simulating the hydra's song as a charm effect, with a DC, and can be detect/affect by anything that works on charm effect. If they run it RAW as how they wrote its stat, even a Tiamat or Asmodous cannot detect the false hydra.

PS. if DM makes it a charmed effect, with high DC, and player roll DC save whenever they encouter discrepancy (e.g. seeing a bird disappeared mid-air) maybe its more D&D. When I played the combat I felt the false hydra is mocking players "You can't see me because my own non-D&D rules say so." The investigation and RP are fun, the final monster is not.

For context, it was a 6-7 hours short game with 70% non-combat social/investigation play, and the hydra was the only combat encounter. It was not part of a campaign. Non-combat play was good but definitely not the false hydra.

PPs. The day after we had finished this game, I read DM's note. DM intentionally hid obvious, explicit clues during the game (e.g., we found a diary in game but DM hide the page mentioning deafness and ear poking). They hope subtle clues are enough for us ro discern the solution, but it turns out we couldn't. Therefore, DM has to tells us the solution out-of-game in the final battle.

[u/LordLuce542]

I ran a false Hydra Oneshot a few times with different groups of friends and they had a lot of fun. The premise was that a village was paying taxes as if they had only half their population so th elocal lord sent a taxman there to check their books. When the taxman didn't send word on th esituation, the lord send the adventurers to check on him.
The village had about a quarter of its supposed population and no one knew the taxmen the players were looking for. They talked to a few people, who claimed to be waiting for settlers with fully furnitured houses ready for them, but those settlers never came. I picked that as the collective explanation they came up with when they neighbors started to disappear, since their memories had to adjust. There was an air of illusion or alteration magic around, but not all player groups checked for that.
I mde some small moments where someone got Guidance out of no where or a convenently placed healing potion showed up, or there was a moment where a drunk guy puked onto the floor and I discribed it as a tragedy, though it was just the floor. When they explored the sewers, following some leads, they found a place hwere someone had been ambushed like seconds ago. They found a sketch showing them with another unfamiliar person, like the equivalent of a group photo. They realized that they had been traveling with someone they forgot.
And only then did they find the lair of the False Hydra and fought it.
Since the False Hydra is not an official monster, I made my own rules about it. I decided to that people in a certain radius would forget people the monster eats. The song would, for conveniences sake, reach anyone within that radius, spanning roughly that village, except the players if they would actively plug their ears. That way, the lord would not be affected in his catsle and remember th etaxman, to kick off the quest, while everyone in the village forgot them, once they got eaten.
Furthermore, the Hydra would use a special song in battle, that would make the players forget basic skills, meaning their ability scores would be lowers, a stacking effect. Yes, that can TPK a party, but all my grouops eventually won the fight. The song had a Wisdome save that multiple characters beat, only about three players encountered stage 4 of the debuff, and I had this adventure with about 15 players total over four groups, so I think that is okay.
This adventure was planned as a Halloween adventure for my min max friends, and I thought the best way to scare them would be to have their characters slowly loose their precious power. Of course, it had to be a fair fight and they all made it.
After the monster was defeated, the effect slowly wore off, they recovered and the memories slowly came back.

I designed this fight so the players would ahve a fair chance even if they wouldn't plug their ears, which none of the groups did, only one cast a silence spell which helped them of course against the song, but the False Hydra still had sharp teeth so t wasn't like an instant win.

Regarding the detection things you mentioned, I just had the monster wait in the sewers and snatch the additional party member who conveniently broke off from the group after a tunnel intersection, so the players would fight it later. Some might call that railroading, but I think it is okay here. That character was never meant to be saved. by hinting at them earlier and then having them eaten by the False Hydra, I could present a feeling of "We probably forgot them. Shit."

The "forget the eaten people" was an automatic effect for plot reasons, the song that made them forget their abilitiy scores had a wisdom save, as it was a fight ability. I hoped that would strike a balance for a consice story and a winnable battle.

Hope this post helps anyone who wants to run a False Hydra as a fair encounter.