Context: at this point, Holmes and Moriarity was (supposedly) dead. Watson is narrating how Moriarity's gang (there had been no evidence linking Moriarity to this gang, despite it being an open secret) got dismantled by the evidence Holmes left behind.As to the gang, it will be within the memory of the public how completely the evidence which Holmes had accumulated exposed their organization, and how heavily the hand of the dead man weighed upon them.
I'm getting a bit confused about the last part. There are 2 dead men here: Holmes and Moriarity. So was he talking about the fact that Holmes who basically used an iron fist and crushed them, or that Moriarity was deeply involved with this gang?
Source: The Memoirs of Sherlock Holmes by Arthur Conan Doyle