Mysterious Ches is a buff in Forsaken World with the following description:
As the description says, you obtain one of the following items when the timer runs out:
Obtaining Mysterious ChestEdit
The Mysterious Chest buff is obtained trough the Shylia Market, on the Favor page -> Regular tab:
You can obtain the Mysterious Chest buff between 20:00 and 23:00. The buff will stay for 30 minutes, after which you obtain the rewards described above. The buff will stay even if you log out and log in the next day.