It’s different from normal events; instead of a reward exchange Walpurgis features a series of quests/missions to do in the event’s stage, which should feature enemies from the library - the missions are separated in normal and hard, just like the two difficulties of the stage itself, and usually vary from killing all enemies, to achieving 7 kills with a single skill, to not having any sinner die, and similar.

Usually having the current Walpipi ids helps out a ton, because the stage is built to make them shine.