King's Test (Star of the Guardians, No 2)

By calling a temporary truce, Derek Sagan and the rebels thwarted the alien Corasian invasion. Enemies once again, the rebels have resumed their defiance and Sagan has retumed to his campaign to topple the corrupt galactic government.  He plans to set up Dion as king of the Starfire dynasty--and to place himself as the ruling power behind the throne.  On a remote planetary sinkhole of sin and corruption, a small weapon-barely ten centimeters on a side--is hidden.  If activated, this seemingly harmless crystal cube could tear a hole in the universe. . .and destroy the fabric of creation.  Sagan wants it.  Lady Maigrey wants it.  And so does Abdiel, a cruel genius who commands a drugged army of mindless slaves. And now Dion is caught in this momentous struggle as he faces his greatest trial yet in his battle to gain the interstellar throne.