It's said that they take a slow poison before they go on an important mission.
The Dark Guards in Thud! tried to invade Commander Vimes's house but unfortunately they came against his butler Willikins who dispatched most them with aplomb. One other Dark Guard fled in panic out of the nursery of Young Sam after seeing something resembling the mine sign of the Summoning Dark on the floor. But he mustered his courage again, and tried to flame Lady Sybil whilst she was in full protective clothing feeding the dragons. The swamp dragons, seeing the flame as a challenge, answered with their own fire. The result was a black outline of the dwarf and two white-hot iron boots.
In Raising Steam, these are the bodyguards and enforcers of the Deep-Downers, who are first seen being used to menace and beat up the Ankh-Morporkian dwarf Magnus Magnusson in the streets of Bonk. They are next encountered as the muscle used by the ultra-conservative grags to impose traditional Dwarf values in Pant-y-Girdl, Llamedos. In this book they are called the Delvers. The expanded Dark Guards appears to be a mixture of the old, highly-trained and skilled, ruthless killers, heavily diluted with the new "Delvers" who appear to be a hastily recruited and patchily trained mixture of volunteers who believe in the ideology, together with dwarfs forced into fighting through threats and intimidation. Willikins would have walked over these and not even noticed. The addition of the "Delvers" is likely due to the slaughter Vimes preformed in the battle under Koom Valley.