I thought it would only be for a few days, just a little hunting trip by myself to escape the other boys' teasing. […] It was quiet enough when I came back, though. The gate stood open. The guard lay across the threshold with his throat cut. Inside, dead, all dead, my lord, my family, betrayed by a hall guest who had opened the gate one dark night to tribesmen from the hills. I tracked that man down. I took my great-grandfather's war axe, which the hall guest had stolen, out of his hand and split his skull with it. His kin hunted me through the mountains half that winter. I killed most of them. Ah, but it was a red, red time.
– Marc, God Stalk, "Chapter 8: Voices of the Past"
Kithorn is an old ceremonial site to the Merikit. The well in the center of the keep leads down the throat of the River Snake, and far predates any keep there. The lord of Kithorn had previously allowed the Merikit access to the keep for their ceremonies.
The ceremony usually involved sacrificing a goat. However, after a season of bad earthquakes, the Merikit planned to pacify the River Snake with a human. The lord of Kithorn found out, and refused to allow the ceremony to happen. The Merikit, fearing the Riverland would be destroyed if their ceremony failed, planned to seize Kithorn, and hold the Kencyr captive until the ceremony was over. They didn't intend to injury any of the Kencyr, however.
A Kendar was guarding the gate, and to get in, a Merikit killed him. Then, fearing the Kencyr retaliation for that, the Merikit panicked, and killed all the other Kencyr at Kithorn to prevent a blood feud.
Marc, one Kendar of Kithorn, was in the hills hunting at the time. When he returned to find what had happened, he engaged in a blood feud against the Merikit.
Kithorn was left in it's half-burnt state, and the Kencyr did not claim Kithorn again. Since then, the Merikit have been able to conduct their ceremonies without issue.
The Kencyr never recovered some of the bodies of those killed in the massacre. Kencyr tradition holds that ever single bone of a body must be burnt, and so they started "bone hunts", searching Kithorn for the bones of the Kencyr. Long after most of the bones were already found, the tradition continued, as a rite of passage.
↑ 1.01.11.2Seeker's Mask, "Part VIII: Kithorn" — " 'This courtyard has been a ceremonial site for time out of mind. The old lord who held this keep didn't mind. He even got his people out of the way so as to give the Merikit a free hand, so he didn't know about all the goats that'd been pitched down his well over the quiet years. The River Snake got 'em all, you see, so the water never suffered. But then there was a season of bad quakes and the Merikit planned to send down a hero. The lord got wind of it, though, and refused to have a corpse thrown down his only well. The Merikit were desperate. They thought, if they didn't do something, the Snake would destroy the entire Riverland. So they planned to seize Kithorn on Autumn's Eve and hold its garrison captive until their work was done. No one was supposed to be hurt.' 'But the Merikit hall-guest who opened the gate cut the throat of the Kendar guarding it,' […] 'So he did. They knew that there would be a blood-price to pay for that, and they panicked. The barracks was sealed and set ablaze; the tower, stormed; the people—men, women, and children, Highborn and Kendar—slaughtered. Tungit wept when he told me, the last time we met before the hills were closed. And then, despite it all, the price fell due, because they'd missed someone.' "
↑To Ride a Rathorn, "Chapter I: An Unfortunate Arrival", I — "For a moment, she might have been looking down on Kithorn, the bones of its slaughtered garrison lying unclaimed and dishonored in its smoldering ruins. None of her generation had been alive then, eighty years ago, but no one in the vulnerable border keeps ever forgot that terrible story or the cruel lesson it had taught."
↑Dark of the Moon, "Chapter 2: The Hell Hunt" — " 'You've probably heard that most of the bodies were recovered the next spring when word of the massacre filtered south. But some couldn't be found. Boys started slipping into the hills on bone hunts. Our grandfathers started it, and our fathers went, too. Now we go, although there's precious little left to find, and we get beaten at home if we're caught at it; but, well, it's become a sort of ritual, and Trinity help the boy who doesn't visit Kithorn at least once before he turns fifteen.' "