In an interview, Cassandra Clare told of when she got her idea for runes.

She said that she had the basis of the story done, but she wanted to make the Shadowhunters more unique.

Her friend had invited her to take a break, and they went to go get tattoos.

Upon arrival, Cassandra saw some black tattoos on the wall. The tattoos were a forgotten writing language long ago.

Cassandra then used each rune for a special attribute, sparking her loved idea of runes.