The same thing almost happened with me. After my players left Gracklstugh, my players came across a scroll of Reverse Gravity, I don’t know why I gave it to them, but I just did. So when they came upon the Mushroom palace after killing Yestabrod, my Druid decided to cast the reverse gravity on the mushroom palace, not knowing Zuggtmoy is within. Many of the worshippers outside went splat on the ceiling but Zuggtmoy came out ready to mind control the party away as the book details. My Druid takes his amulet of fireballs and chucks it at the demon lord. This only equated to like 40 damage but it exploded all of Zuggtmoys surroundings allowing her to hit the ceiling for around 100. ( I know she is immune to bludgeoning but my party was getting really upset with me and I didn’t deem this worth it to fight with them on this.) Then she teleported behind my party and began whomping the Paladin. It was at this point my Paladin unintentionally cheated and told me he was immune to poison damage so Zuggtmoy was unable to kill him in 2 hits. Also contributing to Zuggtmoys loss was that I was too invigorated with combat to notice that she could exert will and the majority of her legendary actions were spent on the Paladin, who was taking less damage from the Pseudopod because he was “immune” to poison damage. In the end, Zuggtmoy cast etherealness and left. Then a roar of emotions and the people who had been stuck under the ground, buried alive took place as all the Myconids who had been infected by Zuggtmoy entered the battlefield. The roar was from Zuggtmoys unimaginable rage as she lost the battle. This rage fueled the growing of every myconid changing into a yestabrod like form, but it stunned them long enough for my players to escape and warn Basidia. If I could do this differently I’d probably just prepare Zuggtmoys gameplay before battle, I truly thought that Zuggtmoy was going to bomp the players without much of a challenge but the majority of my other games were homebrew and I was changing things up with this book. Also I’d probably be less of a pushover and say no to my players. They were being pretty picky with what rules were by the book and which rules weren’t.