Battle(s) of Koom Valley
“Remember Koom Valley!” Two thousand 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).
The Battle of Koom Valley was the only battle in history where both sides supposedly ambushed the other, ending in a stalemate, with both sides endlessly accusing the other of treachery during peace talks. This occurred when the ancient dwarven king B'hrian Bloodaxe came to meet the ancient Diamond king of the trolls, to sign a peace-treaty, ending centuries of warfare between the two races. During the course of the peace talks, hot-heads on both sides instigated fighting that expanded beyond the leaders' grasp. In the midst of the battle, the most relentless rain Koom Valley ever endured stormed the valley for weeks and flooded the valley, quelling the fighting. Due to the flooding, B'hrian Bloodaxe and some of his followers became trapped under the valley with Diamond and some of his trolls. Several of them passed the time playing the game Thud, until eventually all were dead. The bodies can still be found below Koom Valley, encased in stone, as depicted at Deviant Art: Valley of the Kings by Matou31.
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 highly detailed 50-foot panascopic depiction of the battle with annotations in the margins. The Ramkins eventually donated the painting to the Royal Art Museum. When Sybil Ramkin was a schoolgirl she created a scaled-down replica of the painting (see Thud!).