These arm warmers, inspired by the Harry Potter books, are wicked cool. I made them for Rose Love's latest Christmas present, using the same Caron Simply Soft yarn in Dark Sage green to coordinate with her previous Slytherin gear xmas gifts.
This excellent free pattern, Dark Mark Arm Warmers Pattern and Chart, is designed Christa Casebeer of Nerd Girl Yarns.
I opted to knit this all in green and then add the black accents afterward, including a crocheted black picot trim to the top edge and application of the dark mark itself in duplicate stitch (on the left sleeve only!). For such a small applique, I think the mischief of duplicate stitch is a whole lot easier to manage than intarsia or other techniques.