The city lord - Shamin, of the Blue Sky City, relies on his wealth and guards to do as he wishes in the city. Like forcing himself on any females he fancies, destroying anyone who speaks ill on him, etc.
This continued to degrade the condition of the city immensely during 2 years Shamin is in the position of city lord.
So, some well-off families pooled their wealth together and hired an assassin group to kill him.
Shadow's group is the one who took the job.
They had prepared for a whole week before deciding to commence the attack today. Vision and Perceive had spied on the city lord's mansion during that time.
While Review converted whatever they found out into a detailed and intrinsic map. She helped Shadow memorize the map and find the gaps in the guard's defense.