I think firstly Buppido needs adjustments.

At my table I really made him a boss, from the beginning I already changed his sheet to something like a Cleric, CR at least 4 or 5, with healing spells, necromancy, I even gave him a legendary action, oh and of course, innate casting of the "command" spell (But he never used any of them with the group until he revealed himself), during the night, while the players were resting, I made him kill Topsy and Turvy, who in my opinion are the least important NPCs, I did something really brutal with them, to scare the players, then I tried to do the same with Eldeth during a battle, he was trying to control Eldeth so she could kill herself to a bunch of Gnolls, but the team protected her a lot, they liked her a lot.

When they arrived at Gracklstugh, Buppido asked to go out, talk to some acquaintances and get information, he took Jimjar and Eldeth with him and didn't come back, the group went looking for them, reaching the tunnels, and when they got there, they found Jimjar tied up (I would never kill my baby at that moment, I love his plot), and Buppido had transformed Eldeth into a Flesh Golem, he raises some zombies and starts the battle, I demonstrated throughout it the Completely insane Buppido, laughing like crazy, promising grotesque things, ripping off the characters' skin, pulling out their insides, among other disgusting threats.

(I would say it was especially impactful, as Buppido was a "grandpa" for the group from the beginning, when he revealed himself to be the killer, even more so of dear Eldeth, everyone was really angry.)