Mysterious Key is a buff with the following description:
As the description says, you will recieve one of the following keys when the timer runs out:
Obtaining Mysterious KeyEdit
The Mysterious Key buff can be obtained trough the Shylia Market, on the Favor page -> Regular tab:
You can obtain the buff from the Shylia Market between 20:00 and 23:00. After the time runs out, you recieve the rewards described above.