Called Mysterious Exploration, the event offers the chance to win impressive rewards. There's an overall theme of mystery: Production Angels who enter the Mysterious Area have rare materials to collect, while Fighting Angels can gain more in-game money and experience by fighting the Mysterious Monsters. An abundance of new items is also available.
Mysterious Exploration runs from May 15th to 29th.
Don't expect adherence to scriptural tradition, by the way. Angels Online rewrites conventional ecclesiastical history by making Lucifer's fall the result of a jealous crush, and that's just the start of it. We're also not entirely sure what branch of theology allows for angels with robotic penguin companions, but when something's as kawaii as this, the religious implications are surely less important than the fun to be had. Besides, it's free.