This novel is set entirely in the Mirror Universe, introduced in TOS's "Mirror, Mirror", and depicts the aftermath of said episode, Spock's rise to power and his trying to change the fate of his universe.
The overall theme is whether the ends justify the means: Spock's definitely not hesitant about using the Tantalus field to get rid of opponents (no matter how close they are to home), he's exterminating entire species, he's inciting wars, all in the name of bringing peace to the Terran Empire - albeit a delayed peace because he thinks that the Terran Empire can only thrive out of the fire of opression. It first has to be destroyed, the territory and peoples enslaved in order to rise again as a democracy. And why? Because people in power don't want to relinquish it, and the people enslaved don't know any better. Again quite an interesting spin on modern history. But where's the line? Is it justified to throw whole generations into turmoil so that future generations experience a peaceful collaboration and democracy? Spock gives his own answer, well aware of what he's doing.
As usual Mack poses interesting questions, his prose is easy to read and get into. But one point of criticism remains: He's an author best suited to action, to wreak havoc over the whole galaxy, but the little moments, the ones that give you real insight into the characters, the ones that make me empathize, the ones that engage me, that cause me to be invested in a story on a deeper, emotional level... I kind of miss those in his books.
The novel ends with the Empire (or rather the short-lived Republic) conquered by a Klingon-Cardassian Alliance and Spock executed, Spock's legacy remaining hidden within a few asteroids...
... to be continued in "Rise like Lions".