Mystery depots are special items that can be found in Shop -> Mystery depot as seen here:
Each Mystery depot contains several items with various chances of acquisition, please note that you always receive only one of these items per purchase, you do not get all of the listed items. You can view the items inside each box by clicking the "i" button as seen here:
Items inside each box are sorted by their rarity, rarest items might take many Mystery depot purchases until players get them:
Please note that Mystery depots are like lottery, you never know what you get. They are not necessary for in-game progress (except for an optional achievement), if you do not wish to risk getting an unvanted item, for your materials or gems, please do not purchase the boxes as we do not refund Mystery depot purchases under any circumstances. Thank you for understanding.