Here is a video about the Khmer Rouge. Warning: there are a couple of pretty graphic photos for about ten seconds about two minutes into this video from 1:42 to 1:59. The stupas containing the skulls of victims are shown after that point.
Twenty percent of the Cambodian population may have died under Pol Pot’s attempt to enforce a return to “Year Zero” and kill off anyone who had any education or technical training. Cambodia was invaded by Vietnam in 1975, and although eventually Pol Pot was unable to control the entire country, he still operated from the Thai-Cambodian border until finally placed under house arrest by members of the Khmer Rouge who did not agree with his methods. Pol Pot died in 1998.