The Great Transformers
Maybe time and nature? Book T gives its Brief Meaning of Key 13 as "Time, age, transformation, change involuntary..." Hermetic Tarot's LWB states, "The skeleton alone survives the destruction of time and the aggression of nature, slowly working its way upwards." Then it goes on to give the Book T brief meaning quotes above.
This is my guess... I know I find time and age transformative! ;