The Portal rune is a rune "created" by Clary Fairchild which gives her the ability to create a Portal without the aid of a warlock. Her "creation" of this rune was made possible by her connection to the angel Ithuriel.
While in New York, Jace and Clary were discussing their hunt for Valentine Morgenstern. Jace told Clary that they needed to go to Idris to look for him but his request for a Portal was denied. As with past instances, because the need for a Portal arose, Clary soon visualized the image of the rune out of nowhere—this time, on a map of Idris. She drew the rune in midair and the rune was able to create a Portal that shattered into glass after use.