Herbert Rock aka Flagstone was a member of the Myst, who had the ability to transform his body into cement, and also had the ability to shoot liquid, quick-drying cement from his hands.
After he was tasked with seeking out Harry Houdini, he was shot and killed during a fight in the speakeasy of The Grand Canary Hotel by Chanti.
He was described as looking like Bigby from Wolf Among Us.