Arthur could not stay away from the forked pattern that worked its way down Merlin's arm. He would trace it with his fingertips, following all the branches. If he rested his palm on the skin, he would swear he could feel the raised trails. Or he might start with the tip of his tongue, focusing on a main artery.
"Mmm, I like it best when you use your tongue."
Arthur agreed silently and continued, enjoying the tingle that traveled from his tongue to his groin, as though the lightning had left an electrical current in the mark on Merlin's arm.