Corruption

Coen is a man by day and a vrakhir by night, and the game does not pretend those are equivalent. The night form moves better, hits harder, and reaches places the day form cannot. Corruption is what the game charges for that.

Corruption climbs as Coen feeds. Voracious Bite restores him and pushes the meter; so does quenching thirst on the valley's animals, its people and its monsters. As Corruption rises, so does the risk of losing control.

Here is the tension the whole night half of a build sits on: Vampiric Abilities unlock as Corruption rises. The resource that threatens you is the resource that arms you, so there is no version of a strong night build that keeps the meter down.

Shadowstep, Planeshift, Focus, Piercing Shriek and Charge all sit down that road. Planeshift in particular changes how the map works, because a wall stops being a boundary and becomes a route.