Battle(s) of Koom Valley
“Remember Koom Valley!” Hundreds of years ago, the trolls and the dwarfs met in a battle at Koom Valley. It was a remarkable battle in that each side ambushed the other and, of course, in being the historical event symbolizing the enmity between trolls and dwarfs. Nowadays, dwarfs celebrate the Battle of Koom Valley Day, on the same day that trolls celebrate Troll New Year. Sometimes, in a city where both dwarfs and trolls live (e.g. Ankh-Morpork), the two groups are careful enough to plan their celebratory marches so that they are coincidentally on the same street (see Men at Arms).
Some notably intelligent people (e.g. Susan Sto Helit), after carefully studying history books, point out that the Battle of Koom Valley seemed to have been repeated several times. Either the trolls and dwarfs really do hate each other very much (which is, of course, the case), or somebody has been messing with the space-time continuum (see Thief of Time). According to Carrot Ironfoundersson in Thud!, there have been at least fifteen "Battles of Koom Valley", if you don't count the one at Vilinus Pass, which was more of a fracas. Only three of them were in the original Koom Valley.
The most recent "battle" occurred after the detente described in Thud!. As related in the article on Koom Valley in A Blink of the Screen, parties of young Dwarfs and Trolls are taken to the valley by their elders, shown the likely sites, and told never to forget. Unfortunately, one day, two such parties met on the same putative battlefield at the same time and removed all the speculation as to whether a fight may have occurred there. In fact, they made it quite definite.
The Battle has also been immortalized in art: It inspired Methodia Rascal to paint his masterpiece, named the "Battle of Koom Valley" and also dubbed "The Rascal" by art scholars. After visiting Koom Valley and hearing voices, the maddened artist created a 50-foot panascopic depiction of the battle with annotations in the margins. The Ramkins eventually donated the painting to the Royal Art Museum. In Thud! the painting was stolen from the Royal Art Museum. When Sybil Ramkin was a schoolgirl she created a scaled-down replica of the painting.
The post office issued two stamps dedicated to the anniversary of the event. To serve all likings, one of the stamps shows the victorious dwarfs, another the victorious trolls. Suit yourself!