Total Annihilation — George O. Bancroft

Seventeen planets are completely devoid of life when 400 years before they had been on the verge of being asked to join the Galactic Federation. How had this happened? Finding out was the job of the crew of Observer5.
Five thousand years ago these planets had been colonized and left to develop their own unique societies. They had been surveyed every four to five hundred years since they were colonized and many had advanced to the point of being nominated to become members of the Galactic Federation. But now there was not a single sign that human life had ever existed on them. What had happened to them?
That is why Henry Spaulding and his fellow crewmembers of Observer5 are now on the quest to find the cause of this global annihilation. They find a planet its inhabitants call Armont that appears to be in the mid-stage of total destruction.
The last survey had determined that within a thousand years this planet would be ready for membership in the Federation. It would have achieved space flight and would be ready to launch into interplanetary space travel.
But instead of a thriving post-industrial society the crew finds an environment devoid of all of the known lower life forms. Other than the humans who cower in walled cities, the only creatures they find can't be classified by Observer5's on-board DNA database.
There is an immediate culture clash. The team is from the most technological advanced society to populate the galaxy, and suddenly they are endowed with new exotic powers they don't understand, and openly scoff. Jesta warns that learning to use the power of magic is the only hope they have to defeat The Evil One who has taken control of her world and everything on it.
When the smoke clears and the queen is safe, the crew realizes that this was much too easy - it was only an exercise to test them. The real evil power that controls Armont stands ready to test them again and again.
They must fight the beasts to save the people of Comrix, and battle a group of malevolent pirates to free the people of Sycore. But the pirates have sworn allegiance to Gyllu, The Evil One, and are endowed with great power. This malicious horde comes after them to destroy the last bastion of civilization. Not once, but time after time.
And finally it comes down to only one course of action - the crew must find and defeat Gyllu to ensure their survival, the survival of Armont, and the survival of the human race.
Neither Lou or Phil is what they appear to be. While she looks like a young girl in her early twenties, Lou is a scientist who has performed numerous planetary evaluations over many decades. She and Phil have a secret very few people know about - not even Spaulding and Fong. This secret could mean the difference between success and failure for this mission.
And for Philip Bender the secret will mean his own death. Locked in his body is a strange metabolic process that is slowly killing him. Bender is far more than he appears in many ways. He holds the key to the success of this mission and his inner strength will be one of the keys to overcoming the malevolent forces that strangle Armont.