Abbadon is a Greater Demon from Hell. Abbadon is considered one of the Ancients; he is referred to as the Lord of the Fallen and the Demon of the Abyss. He rules the Void—the empty spaces between the worlds and has control over the wind and the "howling darkness."
In 2007, Abbadon killed and possessed Madame Dorothea in her own apartment when she opened the Portal her mother had been protecting. He then hid the majority of its ethereal form just outside the Portal, making him undetectable by the Shadowhunters' Sensor.
When Clary Fray, Jace, Alec, and Isabelle Lightwood came to retrieve the Mortal Cup, the demon confronted the group, wanting the Cup for himself to give to Valentine. In the battle, the demon gravely injured Alec, infecting him with demon poison. Simon Lewis used Alec's bow and arrow to break the skylight, allowing sunlight to burn Abbadon, sending him back to the Void.
Abbadon is said to be at least nine feet tall in its corporeal form. It slightly resembles a rotting human skeleton: it had livid and bruised-looking flesh, a misshapen head, a skull for its face with caved-in holes for its eyes and a nose, two rows of jagged glass-sharp teeth, and black, cracked, and filthy bones that looked as if he was rotting and ancient. His taloned fingers were stripped and skeletal, his thin-fleshed arms pocked with oozing black sores through which more yellowing bone was visible. His hide was gray, with things seemingly crawling inside of it.