Once there were four, but now there's only one: The Voidstone.
For centuries kept secretly by the Sisterhood of Shadows and passed from resolution to resolution, by most it was considered as mythical as the sisterhood itself. But when the last Shadow Sister dies conspicuously in Holy City - Trying to Outdo Gods, it's said - the stone's appearance attracts the attention of three quite different creatures with three quite different motives.
One claims it as ancestral property and a badge of office.
One claims it so as to keep a blood promise to its final keeper.
One claims it in order to heal her lands, and break an ancient curse.
But the dead woman was not the last Shadow Sister - and hers was not the last voidstone.
Set against a backdrop of social upheaval, Voidstone takes the reader from the smoky cliff-bound chambers of the Jade Citadel, where the great and the good conspire to seize an empire for themselves, to the sun-drenched Grand Circles of Holy City, where that empire's death throes finally begin. We see Obsidian City since its gutters ran with the blood of the great families and its palaces burned; we pass by North Shore where the Undying Connective do their awful and awesome experiments; and we walk the lands many call Marash, from where travellers do not return.
We glimpse the past that makes their world what it is, at a point where very different futures are being born.
We follow several people whose lives are changed irrevocably by the voidstone's passage. Suriya, an idealistic young lawyer whose dreams are betrayed by experience. Esuin, an old assassin who remains undying so as to make one final resolution. Amadli, a teenage boy lost in the wilderness who learns about himself and about paying attention. But we also meet Lightworkers, Sentinels, the Undying Connective, the Empire's Rulers, and the Emperor himself.
(It's not 100% necessary to read Shadow Sister first)