Upon cessation of Khmer cities containing their unique Prasat and 2 relics within, the Relics are nowhere to be found in the great works window. Strangely enough, these "phantom" relics show up in the trade window and can be traded with others. This cannot be an intended feature because ceded cities from the English do not lose their additional Archaeological Works/Slots when you acquire them. Also, Khmer will never trade away their relics for anything, valuing them above even multiple cities in a peace deal. Most of your cities? Ok. Only 1 Relic? No.