Worms Of the Earth by Robert Ervin Howard

Bran Mak Morn, King of the Picts, vows revenge on the Roman governor Titus Sulla after witnessing the crucifixion of a fellow Pict. He seeks forbidden aid from the Worms of the Earth, a race of creatures who were once men but after generations of living underground have become monstrous and semi-reptilian. He secures their help after stealing a religious item of theirs from a barrow, trading it back in return for them delivering Sulla to him for a battle to the death.… (more)