Coen is human by day and vrakhir by night, and the two halves are not balanced. The night form moves better, hits harder and opens routes the day form cannot reach. The game charges for that, and Corruption is the invoice.
Feeding raises it
Corruption rises as Coen feeds. Voracious Bite restores him and pushes the meter; so does quenching thirst on animals, humans and monsters. As it climbs, he risks losing control.
And Corruption is what unlocks the tree
This is the tension the whole night half of the build sits on: Vampiric Abilities unlock as Corruption rises. The resource that threatens you is also the resource that arms you. There is no version of a strong night build that keeps Corruption low.
Shadowstep, Planeshift, Focus, Piercing Shriek and Charge all live down that road, and Planeshift in particular changes how the map works, because a wall stops being a boundary.
The day form is not a punishment
Witchcraft is daylight only, and it is not a consolation prize. Compel Soul lets you question the dead, which in a valley this full of them is an investigation tool. Mercurial Fervour slows time and doubles as day-form travel. Hexes cost health rather than a mana bar, which makes them a real trade at any Corruption level.
Swordmastery is the only tree that works in both forms, which is why almost every build leans on it regardless of how the day and night split goes.