The 12th century Cambodian king Jayavarman VII "is generally considered by historians the most powerful Khmer monarch of all time," according to the unimpeachable sources at Wikipedia. So with the Khmer people on their way to Civilization 6, as 2K Games announced today, it only makes sense that he'd be the one to lead them.
"Rising to power during a period of crisis for the Khmer, Jayavarman was a military leader. By 1181, Jayavarman VII had repelled Cham invaders to the north and when hostilities died down, he crowned himself king. But instead of turning outward and seeking to aggressively expand, he focused on his people," 2K wrote. "King Jayavarman VII saw himself as a warrior for his subjects. As a result, his rule was marked by its tolerance and his drive to create a place of safety and paradise for his subjects."
Which isn't to say that the Khmer don't kick ass, and in fact their unique unit, the Domrey, might be the coolest of them all: It's a war elephant with a ballista mounted on its back, making it basically the 12th century equivalent of self-propelled artillery. On the more homefront-focused side of things, the Khmer unique improvement is the Prasat, a replacement for the temple, which gives the Martyr promotion to missionaries it produces and provides a Relic Great Works slot.
Jayavarman himself brings the "Monasteries of the King" unique ability to the game, enabling the Khmer to grab adjacent territories when their Holy Sites are completed, while aqueducts will increase food production in adjacent farms and provide a bonus to Faith and an Amenity through the "Grand Barays" unique ability.
Owners of the Civilization 6 Digital Deluxe Edition will receive the Khmer civ, and the rest of the DLC it comes with, automatically when it goes live. A launch date hasn't been announced, but I would expect it to show up at around the same time as the big Fall Update.