Night Kill was one of the rare upgradeable specialties.
A Night Walker’s fighting ability during the night would fiercely increase as this specialty leveled up.
But leveling this specialty would take a long time. Last time it happened was when Marvin took over the Southie. So much time passed in a blink and since then Marvin had killed countless creatures during the night. Ultimately, killing those two Dark Side Vampires leveled up the specialty.
After it leveled up, its description changed as follows:
[Night Kill (Hidden Specialty - Activated)]
Type: Passive/Upgradeable
Level: 2
Effect: During the night, Attack Power +6%, Attack Speed +6%, Movement Speed +6%, Burst Power +6%, Reaction Speed +6%.
...
This was a simple and rough passive!