Look to Windward - Iain M Banks (Book 7 of the Culture series)

It was one of the less glorious incidents of a long-ago war. It led to atrocities on an extraordinary scale. To the destruction of two suns and the billions of lives they supported.

Now, eight hundred years later, the light from the first of those doomed stars has reached the Culture Orbital, Masaq', bathing its ­fifty billion inhabitants in the rays of their society's ancient mistake. Amongst them is Major Quilan, sent on a mission so secret that not even he knows what it is, and determined to exact his revenge against the Culture at any cost.