The first and most simple rule of the Darkness is that it must consume all Light. And how better than from the very souls of the Light's servants? The character reflexively senses the presence of all Light-touched characters and objects - Blessed Places, Nobles, Shikigami, Sworn, Beacons and Bequests - in his immediate proximity. Moreover, by concentrating (an instant, unrolled action) he can locate and identify any such beings that are present. Powers that conceal a Light-touched being’s nature defeat his passive sense automatically, but concentrating starts a Clash of Wills with such powers.
Umbra: Recognizing the Light at work inspires the character with deep hatred which he cannot conceal. If the character does find a Light-touched being with this power, he must take the Obsession Condition [CofD 290]; he is obsessed with destroying the person, place or object that the Light has touched, by whatever means come to hand. This power is responsible for many of the Darkness's most cruel attacks.