This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Interesting Times
The death of Twoflower's wife. It is the only time Rincewind has ever seen Twoflower look anything other than happy, contented and trusting. Later, of course, he challenges the villainous Lord Hong to a duel to the death as payback — which is also highly out of character for Twoflower. And as we all know, OOC Is Serious Business.
"Rincewind: 'I was, er, I was sorry to hear that your wife, eró'
Twoflower: 'Things happen in war. I have two dutiful daughters.'
Rincewind opened his mouth to say something but Twoflower's bright, brittle smile froze the words in his throat."
The fact that Twoflower's wife was basically caught in a crossfire just makes it even more heartbreaking.
. Having spent virtually all his life as The Ace in whatever he sought to do and essentially controlling the Empire by proxy, Lord Hong's ultimate goal is to rise to power in Ankh Morpork and play chess with the Patrician. He even spent months having spies travel to the city, carefully ordering specific pieces of clothing until he had a complete set of clothing that fit the description of a Morpork noble. Amidst his detachment from virtually everything else, Hong literally trembling with anticipation as he periodically tries on the clothes is...heartbreaking. For all the evil he's done, he practically becomes a tragic villain when you realize that he's been The Ace ever since he could walk and talk, but it doesn't matter when the populace of the Empire will grovel at the feet of anyone with a horse. He wants nothing more than to rise to power in a city where he has to earn his respect by his own merits.
When you think about it in those terms, dying before ever getting to Ankh-Morpork is the best possible thing that could have happened to him, because the book points out that there's no way he could get to the top solely on his own merits. Being The Ace in a culture that has stagnated as much as his has wouldn't translate into the practical skills necessary to survive in Ankh-Morpork. He'd have to buy his way in which is is exactly what he wanted to avoid.