Surface Detail - Iain M Banks (Book 9 in the Culture series)

The "War in Heaven", a simulated war game, rages between civilisations. Its virtual battles have been fought for decades, and the victors will decide the fate of the digital Hells - torturous artificial afterlives with horrors beyond imagination. In the Sichultian Enablement, Y'breq is one of the Intagliated, her marked body bearing witness to a family shame, her life belonging to a man whose lust for power is without limit.

As the virtual war threatens to spill into the Real, Y'breq is willing to risk everything for her freedom - but she'll need the Culture, and its help comes at a price. The Culture is going to war with death itself.