According to legend, when Genghis Khan died in 1227 in what is now northern China, his lieutenants wanted to keep the death a secret from the Mongols’ enemies. So as the party accompanying his body made its way back to Mongolia, they killed every person they saw on the way – more than 20,000 – so news of the death wouldn’t spread. Then, when they buried Genghis, they either redirected a river to cover the site, or set horses to trample the ground so no trace would be seen, or killed all the people who buried him, and then killed those killers.
There is no hard evidence that any of those things happened. It may well be that they are after-the-fact embellishments designed to explain a remarkable circumstance of history: the location of Genghis’ tomb remains a mystery.