Shadow Lands by K.F. Breene — book slip

Shanti is determined to claim her title as the Chosen and lead the Shadow People out of the Land of Mist. To do so, she must survive The Trials, a centuries-old system designed to test her battle awareness, fighting skills, and survival instincts. Alone on a dangerous island, she faces a race against time and a sinister force rigging the trials.

Meanwhile, Cayan is separated from Shanti and must protect his men from the Inkna and Graygual who flood the island. As he learns to harness his mental skills, he discovers a deadly presence in the woods. The battle to become the Chosen has begun, and the outcome is uncertain.