Quintara Marathon #2: The Run to Chaos Keep by Jack L. Chalker

Many different beings in the galaxy shared one legend: demons. When an expedition to an unexplored planet discovered two gigantic horned creatures in suspended animation, they realized, just before dying horribly, the truth behind the legend. Rescuers answering a bizarre distress call found what seemed to be an entrance into another dimension, and had no choice but to pursue the demons into an insanely twisted space. And the desperate pursuers would find that the legends of demons being a danger to one’s soul was as much a reality as the demons themselves.