WARNING: The following contains spoilers for Shadows House Episode 11, "The Dark Drink," now streaming on Funimation.
The centerpiece of Shadows House, the much-feared Debut, is now over. Nearly every Doll/Face pair managed to solve Edward's twisted tests and emerge victorious, ready to be formally introduced to the society of Noble House. Sadly Rum, the timid Living Doll, found and freed Shirley from her poisonous cage, they never made it out of the maze in time.
In fact, as Edward had predicted, Shirley was too weak and collapsed, her entire body reverting to soot even as Rum tried to guide her to the exit to get some help. It's a sad and horrifying end for the two, so why doesn't anyone seem to care?
Shirley's failure is announced in the most brutal way in Episode 11: a cautionary tale against weakness of any kind, while the others were celebrating their success. The wooden doll that represented Shirley through the Debut is violently thrown to the ground and shatters. No one even mentions Rum -- not even sweet Emilico or stalwart Shaun, who had been her best friends up to that point. Everyone had already taken a sip of bitter coffee laced with soot, so the only thing their minds can process is blissful loyalty toward Lord Grandfather, the mysterious patriarch of the mansion.
While the tampered coffee doesn't affect the Shadow children in any way, its effects manifest in two different ways for the humans. First, their eyes are clouded by dark, frantic scribbles, losing their usual color and expression. They lose all sense of individual thought, with their minds bending to the dictates of Noble House: "do not fret about trivial things," "your loyalty is to your Master and to Lord Grandfather," etc. to the point where they explode in tears of joy at the glimpse of Lord Grandfather's hand.
Kate is the first one to notice this change in Emilico, and it immediately freaks her out. She loves Emilico not because she's a perfect copy of Kate, but because she can think outside the box, has an extremely positive personality and is headstrong. The soot-laced coffee has erased those traits, although her devotion for Kate remains. So she decides to take matters into her own hands and purge the soot from Emilico's system.
She finds the cure in Emilico's "Do not Fret" notebook: water, water and more water, enough to almost be considered torture. Emilico, wishing to please Kate, drinks it all. After vomiting up the last of the soot, she finally remembers what happened to Rum and Shirley. She's horrified that even now, after regaining her senses, she can only feel joy about it.
So why are the Adult Nobles drugging the human children? While the first dose of soot erases their memories and makes them more pliable, subsequent drinks only turn them into soot addicts and actually prevent them from properly bonding with their masters, which can be fatal for both Shadow and Living Doll during the Merging -- and nobody in Shadows House wants a failed Merging.
The main point of the soot is to make everyone in Noble House intensely loyal to one person: Lord Grandfather. However, his powers do not affect Shadows, only humans. From his perspective, a successful Merging is one where the Shadow child not only takes over the body of the human but where the body itself, conditioned by years of soot consumption, turns the independent and powerful Shadows into his own loyal minions. The risk of losing a couple here and there is worth it, as long as he keeps a mansion full of powerful magic users.
Kate quickly shares this information with John, who decides to forego the water treatment and just knock Shaun until he recovers his senses. However, Patrick and Louise do not care that much about the secondary effects of the soot -- though Patrick might have appreciated a respite from his scheming, domineering Doll Ricky. Louise, meanwhile, seems to have inherited Lord Grandfather's mind control powers -- by blowing soot into Lou's mouth, she's able to completely control her emotions. The entire devious plan within the mansion is becoming clearer as Shadows House continues.