The essence of change
According to the Book of Martin
The essence of change is the dust of the world which when manipulated through thought, action, or device transforms the elements.
Magic is the method by which the essence of change is directed.
Corrupted essence or corrupted magic is heavily influenced by desires and emotions of the environment around it and often performs in unintended and undesirable ways.
Taming corrupt magic is a complex practice requiring a device called a shackle to transform it.