Complex sentence: They are the remenants of the original invading population who were mostly wiped out in a war with the K'Chain Che'Malle, living in hiding for thousands of years and ruling the human population from the caverns under the city itself.
Split into simpler sentences: They are the remenants of the original invading population who were mostly wiped out in a war with the K'Chain Che'Malle. They have lived in hiding for thousands of years, ruling the human population from the caverns under the city itself.