They were lying in bed together, bare and ruddied after engaging in a second round of forbidden desires. Sarah idly stroked her lover's body. Her wandering hands found  a small spot on her thighs. 

"Mistress," she asked, flirtily. "What is this? Have I found thy witch's mark?" 

The older woman smiled back at her. "Yes, comely Sarah. That was the mark the devil bade give me when I pledged myself to his service. It shows his ownership of me. That's how I was able to gain the power I use to bewitch thee." 

Sarah let herself a puzzled look hide her mischievous face. "Ah but mistress - if such a mark shows ownership, then why do I lay so unblemished? For truly I am owned as thoroughly by thyself as one could ever be owned." 

Her mistress smirked. "Thou art a wise poppet," she replied pushing her back into the bed where they resumed their beastial romps. 

The next day, Sarah walked through the town being even more careful than normal to cover herself modestly (and thoroughly). Occasionally, she touched her neck. She shivered at the slight pain from  the bruise her mistress had left last night. It felt right.  She wished she could show the town although she knew that could not be. Still- she was proud to now carry her own new witch's mark.