- The Summoning Dark.
- The worst part is how it's clearly gaining more and more control over Vimes in hindsight, even though he doesn't seem to realize it. By the time he reaches Ardent and the other grags, he's barely seconds away from killing them, and if not for his inner Watchman, he would have.
- And In-Universe, Vimes possessed by the Summoning Dark rapidly becomes this to the dark dwarfs at the end.
- "A short dark figure was at the top of the stairs and disappearing into the nursery."
- "They killed my son." "I'm sorry, commander, but it's true. I don't think Gunder Ironcrust was interested in the politics, you see. He just took a job at the mine because he wanted to feel like a real dwarf and work with a shovel for a few days." "They left him to the mud...any help you need, we will give. Any help. But when you find them, kill them all."
- The sheer horror of what unleashed the Summoning Dark; the fact that one of the dwarves who was so cruelly betrayed managed to get to the door and the chance of escape — only to find it barred, and wore himself out clawing at it. Then he drew a curse in his own blood and sat down to wait for death, with the vurms slowly approaching. Meanwhile Helmclever was on the other side of the door, too afraid to open it and wishing the other dwarf would die quicker.
- Helmclever's death, if only for how quickly and quietly it happens. One second, he's alive, and then Vimes accidentally knocks over the candles. A few seconds of darkness later, he's dead, and the Thud pieces on the board have formed the shape of the Summoning Dark. All because Vimes lost his temper at Colon. And while he certainly didn't touch Helmclever, he didn't touch the dwarf that went after Sam, either...
Nightmare Fuel / Thud!