The Farpool: Union

Six thousand light years from Earth, the star-sun Sigma Albeth B has gone supernova, the result of a Coethi weapon called a starball. Five worlds were destroyed, including Seome. Millions died but twenty thousand survived, escaping through the Farpool to the oceans of Earth. Chase and Angie find themselves in the midst of unending conflict, between the water clans themselves and between the emigrants and their unwilling hosts on Earth. A few want to return to their home, through the Farpool to a time before the End Time. They prevail on Chase to help them.
Chase leads an operation called Temporal Hammer back to Seome, using the Farpool as a time machine, to try and assist the humans of that time in defeating the Coethi and preventing Seome from being destroyed. But he and Angie end up trapped in a civil war among the kels and a war of succession inside one of them. The Seomish have disabled the Time Twister that the human forces are using to defend their sector of space, unwittingly laying their home world wide open to destruction. Captured and charged as a spy in this civil war, Chase finds that the only way he can undo what has been done is to engage the Seomish on their level, fighting for his life in a blood-sport called tuk. If he wins, the Seomish will let him go and help him re-build the Time Twister, allowing the humans to defend their sector of space, saving the world from obliteration. But if he loses, the time stream will play out as before and millions will die.
Fifth and final title in The Farpool Stories.