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Tear Jerker: Thud!
Any time Sam Vimes thinks about being a father counts, especially during the dwarves' raid on the manor: "I'll kill I'll killyoukillyoukillyoukillkillkill" Gives me chills.
The above scene is particularly chilling and tear-inducing if you're a parent.
"So, that was the rule. Six o'clock, on the dot. No excuses. He had nightmares about being too late. Actually, he had a lot of nightmares about Young Sam. Most of them involved empty cots and darkness."
The above becomes even more Fridge Horror when you realize that the only reason Vimes was home to save Young Sam was that he'd made it a point to get home by six o'clock that day, even though everything in the city was falling apart.
The sheer, unadulterated beauty that is the Guarding Dark, whose existence essentially confirms that Vimes has a Crowning Psyche Of Awesome.
"Call me... the Guarding Dark. Imagine how strong I must be." Does the trick. Every. Single. Time.
When Vimes falls into the underground river and Sybil goes to read to little Sam. Followed by the mini-tearjerker cutaways back to them during Vimes berserker rampage.
When he's lost in the caves, brought to a beserker rage by the Summoning Dark, Vimes still manages to focus on his love for his son. THAT! IS! NOT! MY! COW!
Added to that was Sybil's quiet and never ending belief in Vimes. You can tell she's worried and scared but she sits calmly and believes in him. It's never said, but you can tell when she finally goes to little Sam, she's scared the worst has happened.
"He will walk through walls!"
Young Sam's reaction to Dad not showing up for reading time does not help. He's obviously far too young to understand what's going on, but he knows that something is wrong, because Dad never misses reading time.
Especially when afterwards, Vimes puts 'Helmclever was terrified that the Summoning Dark would come for him' together with 'I was the champion of the Summoning Dark' and is immediately terrified that he killed Helmclever, before Bashfullsson reassures him that he didn't.
The deaths of the four mining dwarves, especially Setha Ironcrust's son. The poor fellow, the son of a renowned baker, just wanted to feel like a real dwarf for a few days, working for grags, digging down in the dark. And they got left for dead in the cold and mud for overhearing a secret. Vimes has a mini-BSOD when he realises he was on the other side of the door from the dwarves.
Bear in mind (though nobody in the room knows it), when Ironcrust asks Vimes of nothing but to track down the grags responsible and kill them, Vimes is the Champion of the Summoning Dark, the demonic avatar of vengeance. The fact that Vimes is then able to find the grags and not slaughter them speaks volumes more of Vimes' willpower.
Also, when Angua reassures Vimes that the four dwarves were certainly dead (and not just dying) when they were wandering around in the mine near the area where they were killed. In a previous scene, she had confessed to Carrot that the dwarves had quite possibly still been alive at that point. She lies to Vimes because she knows the truth would probably kill him. Or, at least, make him kill someone else.
The sheer tragedy of the truth about the first Battle of Koom Valley. It was originally a meeting for a peace accord, but some of the representatives fell to fighting after a sudden fog lifted. Everyone involved got trapped in the caves beneath Koom Valley after a flash flood, and the messenger sent to bring word of what happened died before he could bring someone to the Cube. In the end, what was supposed to be the start of a new era of peace just became another excuse for both sides to hate one another. B'hrian Bloodaxe and King Diamond would have rolled over in their graves, if they had ever gotten graves.
The touching discovery that Diamond was playing the dwarf side in the Kings' Game. That water has been quietly dripping on stone, wearing it away bit by bit, for a long, long time.