Because of its location far north, Evernight City was -as the name suggests- dark for the better part of every year. Not to mention that it was also very cold, wintery, and snowy for an equal period of time.
For newcomers, the city was gloomy, dark, and yet also beautiful in its own right. In rare cases, darkness might exude charms untouched by any other aspects and Evernight was a prime example of that.
The people who lived in this place were characterized by their noticeably paler skin, probably from the lack of sunlight.
Anyway, Evernight, being a dark city -literally- was completely shrouded with a dark veil all day every day, adding a layer of mystery to it. Perhaps that was the reason why the Hitman Guild chose this place of all places.
The founder must've thought that the darkness of this city would hide the darkness of his Guild. In any case, the headquarters of the Hitman Guild was built there.