The Death Zone — E.C. Tubb

Earl Dumarest accepts a commission to guard Lady Derai, of the House of Caldor, on the feudal world of Hive. But there, his hopes of meeting a living witness to planet Earth are dashed. The man, along with his fellow villagers, has mysteriously disappeared. Meanwhile, Central Intelligence aspires to universal control and the Cyclans are their emotionless servants. When they bring Dumarest into the deadly Contest on Folgone, he's forced to fight alongside Derai to escape the death zone.