What happened between 800 and 1200 in khmer´s territory?

The Khmer empire was a powerful state in South East Asia, formed by people of the same name, lasting from 802 CE to 1431 CE. At its peak, the empire covered much of what today is Cambodia, Thailand, Laos, and southern Vietnam.

By the 7th century CE, Khmer people inhabited territories along the Mekong river -the world’s seventh longest river - from the delta to roughly the modern Cambodia-Laos border, plus the region between that river and the great Tonle Sap lake to the west and the area running along the Tonle Sap river (which runs from the lake to the sea, joining the Mekong in the delta). There were several kingdoms at constant war against each other, with art and culture heavily influenced by India due to long established sea trade routes with that subcontinent.

I hope this helps