Mystery Symbol
When one or more Mystery Chests land on a reel they lock and begin to clone themselves vertically on each spin until they cover the whole reel while revealing a new symbol on each spin.
When there are one or more winning combinations, the winning symbols will cascade and the new symbols will take their place and the trail will increase by 1. For every symbol which forms part of a win line, the associated reel will increase in height by 1 per symbol on each cascade.