Bonus Chests is a Treasure Hunter promotion originally from February 2015. Bonus Chests contain double or triple the prizes of a regular chest that players can preview before claiming.
During this promotion, the bonus chest is added in the middle of the five regular chests. There is also a meter displaying the charge on the bonus chest. The meter gains one piece of charge for each key used, and five are required to open the bonus chest.
At the start of the promotion the meter has five charges and the bonus chest must be opened for three medium prismatic lamps before any keys can be used.
The bonus chest shows its contents before opening it (even when there isn't enough charge to open it), and its contents change upon each key used and after midnight each day. The bonus chest always contains two or three of the same reward. The extra items which can't be owned more than one of are converted into coins automatically. Three rewards are much more common than two; the chance for three rewards instead of two was doubled on 12 July 2017 from approximately 40% to 80%. Once the bonus chest is opened, five pieces of charge are subtracted from the meter. Charge can be stored up to ten units, and opening chests at that point still keeps changing the contents of the bonus chest.
Opening the bonus chest doesn't use a key.
During this promotion the animation for the rewards from the regular chests is skipped.