As the fog rolls in, Frankland appears. He has leaked the gas again to finish Henry off. In the chaos, a common stray dog (enhanced by the gas to look like a monster) is shot by John and Lestrade. Frankland flees but wanders into a Baskerville minefield, meeting a fiery end. The Aftermath
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 [S2E2] The Hounds of Baskerville
The story closes with a chilling realization. Sherlock’s "Mind Palace" vision of in a prison cell was actually a lingering effect of the gas—a shadow of his greatest fear. As Henry finally finds peace, the episode ends with a teaser: Mycroft releasing Jim Moriarty from a holding cell, where the villain has written "SHERLOCK" all over the walls. O.U.N.D. project? As the fog rolls in, Frankland appears