The hydrogen sonata / Iain M. Banks.

Enhanced descriptions from Syndetics:

The New York Times bestselling Culture novel...
The Scavenger species are circling. It is, truly, provably, the End Days for the Gzilt civilization.

An ancient people, organized on military principles and yet almost perversely peaceful, the Gzilt helped set up the Culture ten thousand years earlier and were very nearly one of its founding societies, deciding not to join only at the last moment. Now they've made the collective decision to follow the well-trodden path of millions of other civilizations; they are going to Sublime, elevating themselves to a new and almost infinitely more rich and complex existence.

Amid preparations though, the Regimental High Command is destroyed. Lieutenant Commander (reserve) Vyr Cossont appears to have been involved, and she is now wanted - dead, not alive. Aided only by an ancient, reconditioned android and a suspicious Culture avatar, Cossont must complete her last mission given to her by the High Command. She must find the oldest person in the Culture, a man over nine thousand years old, who might have some idea what really happened all that time ago.

It seems that the final days of the Gzilt civilization are likely to prove its most perilous.