The Wounded Frontier. Jack Commer, Supreme Commander, #5

When a star thirty-four light-years away vanishes, leaving the infrared signature of a Dyson sphere inexplicably built in one week, Supreme Commander Jack Commer readies the untested Typhoon V for the star Iota Persei, roping in a talented replacement engineer doubtful of Jack's command capabilities, and cajoling a navigator beset by decades-old combat trauma into postponing his retirement for one last risky mission.
Meanwhile Jack's previous engineer, who quit the Space Force to become Consort to the Martian Empress, begins to remember that he's a really a spy for the Wounded, a race that kills suns to create spectacular quasar artworks fifteen billion light-years away.