"An honour it will be then. Come, put out your left arm for me." He smiled, preparing to bestow his reward upon Isabel. As a matter of fortune, Bellatrix carried a knife on her person at most times, and he withdrew it. Without a second thought, he knicked himself over Bella's mark, letting a few droplets of red blood fall onto Isabel's arm. Leaving the wound open- he was certain that, when his body was returned to its owner, she would find joy in a wound inflicted upon her by him- he drew out Bella's wand, pointing it at Isabel's forearm. A moment of concentration, and... "Morsmordre!"
The blood was drawn into Isabel's arm through the skin, and in its place the familiar skull and snake rose in black, standing out strongly against the pale skin.
"You are now one of my chosen. You are marked until the end of your life, and your loyalty won't waiver."