Quintara Marathon Series by Jack L. Chalker

On an uninhabited Earth-like planet, a scouting expedition uncovers the perfectly preserved bodies of horned creatures, and the three primary cultures, maintaining a precarious balance of power, vie for control of the mysterious aliens.

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... moreMany 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. less

Three teams pursue horned creatures known as demons through a gate into another spacetime continuum, and fight each other all the way. But they now must join forces to save the 90 trillion beings of the gallaxy in danger of losing their lives--and souls.