In a world plagued by demonic spirits known as Ethers and Umbras, the Arbiter is the only person capable of defeating them. You play as Ashe, the new Arbiter, whose job is to venture out and defeat those that threaten humanity. The Arbiter has the unique ability to transform between Ether and Umbra powers, allowing him to fight back against the spirits that hunt him. An Ether can only be defeated by an Umbra, and an Umbra only by an Ether. By mastering transformation, you can manipulate how these powers interact to complete your mission.
Here's some short gameplay showing off the first boss in the game, Wolf!