Initiation.

The skins of the recruits became the recipients of the attention of the burning leaves. The leaves licked their skin and burned it like any fire would do. But they don't burn haphazardly. The wounds they leave behind form black lines that look like the drawings of a chain. 

The process of initiation is very painful. Fire licked their skin to form patterned injuries. The dormant law fragments in their skin were activated with divine energy. The divine energy of Order was used so the law fragments were reinforced instead of being completely destabilized. 

The process of reinforcement was accompanied by the destabilization of the divine energy of the sun. The law fragments were broken down by the divine energy of the sun and then reshaped with the divine energy of Order. So it is like a metal smith using heat and some extra metal to weld pieces of metal together. All of these led to the creation of simple black lines on their skin.