Short story of Clary and Jace.

Cassandra Clare's description on Tumblr:

"This was written on request as a gift for a friend who did something kind for me: it is, however, canon with the books and takes place while Clary and Jace are on their mission in Faerie in Lord of Shadows. (If you squint, you can even guess when.) Clary struggles with what it means to love and keep secrets, and sexy times are had. Definitely R-rated: so, warnings for sexual content both visual and textual.

Some spoilers for Lord of Shadows; Clary and Jace are about 22/23 here."