The Earthlords by Gordon R. Dickson 1989
SOME CALL IT HELL...
Deep in a hidden labyrinth beneath the Canadian wilderness dwell dwarfish Earth Lords and Ladies who ride humans like horses - and plot the final downfall of mankind.
Bart Dybig is a "Steed", but one gifted with mental and physical abilities unsuspected by those who have enslaved him. Soon, he vows, he will surprise the Lords and escape to the world above - if there's a world to go back to.
For the Earth Lords are building a doomsday device of unimaginable power to completely destroy mankind. Only Bart and his strange heritage can stop them...