Sherlock realizes the drug isn't in the sugar or the water—it’s in the of Dewer's Hollow.

The chemical is a volatile, aerosolized fear-gas developed at Baskerville (Project H.O.U.N.D.). It targets the brain’s fear centers, turning a suggestion—a "hound"—into a vivid, lethal hallucination. Sherlock’s Mind Palace reveals that "H.O.U.N.D." was an acronym for the scientists who created the gas in the 1960s. The Confrontation at the Hollow

They rush back to the Hollow to find Henry on the verge of suicide, driven mad by the gas. Sherlock explains the truth: Henry didn't see a monster twenty years ago; he saw in a gas mask and a "H.O.U.N.D." t-shirt, murdering Henry's father over the secret of the gas. Henry’s child-mind interpreted the mask and the red goggles as a demon dog.

That night, Sherlock and Henry venture into Dewer’s Hollow. In the darkness, amidst the swirling mist, they see it: a massive, snarling beast with glowing eyes. The Great Detective’s Doubt