In Her Name by Michael R. Hicks

Led by Commander Owen McClaren, the TNS Aurora is embarked on an extended survey mission, searching for new worlds that could support human life. Drawn to an uncharted star system by the discovery of potentially habitable planets, the crew of the Aurora discovers something entirely unexpected: the planets are already inhabited, but not by humans. Approached by gigantic alien starships, Aurora's crew makes ready for humanity's very first contact with another sentient race.
But nothing could prepare them for what fate has in store. For they have entered the domain of the Kreelan Empire, which has waited thousands of years to find another spacefaring race against which to wage war to honor their Empress. With all but one of the crew killed in bloody close combat, the aliens send Aurora home bearing the sole survivor: the Messenger, a young crewman who carries with him an alien artifact that is humanity's only sign of how much time remains until they are plunged into an interstellar