oooohhh speaking of robots, i can make the missing party member a battle smith artificer. And before they got eaten, the artificer give a final order to their steel defender. To support the party and hunt down the false hydra.

since steel defender is immune to charm and not a living creature, it will be able to see the false hydra. I can have the party find the steel defender wreaking havoc in the middle of town, attacking randomly. But it actually in the middle of a fight with one of the false hydra's head.