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. However, Sulla's mind is broken from his contact with the horrific Worms of the Earth and Bran Mak Morn slays him in mercy rather than vengeance, realising that some weapons are too foul to use, even against Rome.