After Will made love to Tessa Gray in 1878, the latter's clockwork pendant ended up resting next to his collarbone while they were asleep. The pendant had the angel Ithuriel trapped inside and it burned a pale white star-shaped scar on Will's shoulder. Will's son and their male descendants all then inherited the mark.
Apparently, this did not happen by chance; the angel in fact decided to mark Will, partly due to their connection to Tessa—Ithuriel's feelings about her and how she, in turn, felt about Will. Additionally, the mark is passed down the male line of the Herondales possibly because that's how Ithuriel meant for it to work, as a mark on a family it may have wanted to keep track of. "It marked Will because of love, death, guilt and grief. There are no genetics here: there is only magic: the magic of magic, and the idea of a family being marked forever by a specific love being made concrete.", even Magnus found it "interesting."
The story, however, that has reached the later generations, is slightly different from reality. According to Amatis Herondale, who was once married to Stephen Herondale, an old legend in the family says that an angel came to the first Herondale who became a Shadowhunter in a dream, during which the angel touched his shoulder. That contact then manifested as a mark that appeared on the Shadowhunter's skin when he woke up. They believe that it means that the bearer is blessed in some way.
There are still some truths to this; in a sense, Will can be considered the 'first' Herondale to become a Shadowhunter after his father left the Clave, and his shoulder did come into contact with an angel in his sleep, albeit through a pendant. Whether or not Will in fact saw Ithuriel in his dreams is unknown.
- There are two other people not related to the Herondales who have similar scars: Jocelyn and Clary Fray. They received the mark presumably because of the angel blood, also derived from Ithuriel, that Jocelyn received from her husband, Valentine Morgenstern, while she was pregnant with Clary.
- ↑ 1.0 1.1 Clockwork Princess (chapter 19)
- ↑ 2.0 2.1 City of Glass (Epilogue)
- ↑ "The Herondale scar works like the Herondale name, anyway. It is passed down from Will’s generation, only to the boys. The girls don’t have it." - Cassandra Clare on Tumblr
- ↑ "It marked Will because it wanted to." - Cassandra Clare on Tumblr
- ↑ Clockwork Princess (chapter 20)
- ↑ City of Bones (chapter 17)
- ↑ City of Glass (chapter 17)