How did he get there?
It wasn't on purpose, but everyone in his position experienced the same thing as long as they gained their power through proper means.
Damien eventually decided to try and summon the energy of Nonexistence. He couldn't control it and he could only slightly perceive it, but it did exist in his body.
It was an aura that manifested in the way he wanted it to. For most people, it would turn black, but that was a color that represented the Void in Damien's mind.
Existence was a formless power. Damien viewed Nonexistence the same way. As such, when he summoned it into the cave abode, it didn't necessarily appear.
The air became several degrees colder. Not because the temperature was decreasing, but because the air was being banished from the room.