After skipping Civ V, the Khmer are returning to the series in an update for Civilization VI.
They’re led by King Jayavarman VII, and their perks include a religion bomb (their temples will claim adjacent land) and war elephants.
The Khmer are coming soon as paid DLC (unless you got the digital deluxe version of the game, in which case you’ll get them for free), along with a whole bunch of other changes and improvements.