Dark Chains: Second Link by I. T. Lucas

She was his childhood best friend. Now she is the woman he would risk everything for, even if it means breaking the connection that keeps him alive. The three-meter concrete walls of the compound are not the biggest obstacle keeping them apart. To love Sullha, Yaaf has to become the boy she once knew, but to save her, he has to remain the weapon he has become.

Meanwhile, across the world, Kian is finalizing a plan built on precise timing. The five chests must be removed from the hidden chamber before Losham’s brothers discover what lies inside them, and the success of the operation depends on eight enhanced soldiers and two Russian scientists he has no reason to trust.