Three Dark Crowns by Kendare Blake

Three sisters born on the same day are raised apart as candidates for the crown and must fight a ritual duel to the death until only one remains; each wields a different, deadly kind of magic—one commands the elements, one masters poisons, and one draws power from nature and beasts—and each is groomed by competing factions. As training, betrayals, and political scheming escalate, the contest becomes a ruthless struggle for survival, power, and the future of their fractured kingdom.

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