The Hydrogen Sonata (Culture #10)

They are, truly, probably, End Days for the Gzilt civilization.
An ancient people, they helped set up the Culture ten thousand years earlier & 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 & almost infinitely more rich & complex existence.

Amidst preparations though, the Regimental High Command is destroyed. Lieutenant Commander (reserve) Vyr Cossont appears to have been involved, & she is now wanted--dead, not alive. Aided only by an ancient, reconditioned android & a suspicious Culture avatar, Cossont must complete her last mission given to her by the High Command--find the oldest person in the Culture, a man over 9000 years old, who might have some idea what really happened all that time ago. Cossont must discover the truth before she's exiled from her people & her civilization forever--or just plain killed.