Gabriel Lightwood and Cecily Herondale were assigned by Charlotte to procure ingredients for Magnus Bane from Sallows. Sallows recognized the pair as Nephilim—Gabriel with his rune marks and Cecily through her resemblance to Will, who was also a regular customer when he was running errands for Magnus.
When he learned that Gabriel was the son of Benedict Lightwood, he, unaware that Benedict had died, produced some of his fathers orders, particularly what seemed to be a demonic Kama Sutra of sorts, for Gabriel to pay for and take home. Insulted, hurt, and humiliated, Gabriel threw Sallows out of the window, chased him down the street and hurled him into the canal, though Gabriel claimed that he did this because he showed inappropriate images to a lady.
Gabriel went on to take the parcel of Magnus' ingredients and other objects from his shop. A frustrated Sallows later returned to his shop and was fixing his windows when a hooded figure came in to the store. Assuming it was Gabriel again, he called him out as a Nephilim and threatened him with his "powerful friends". The man later took out cold iron, a noted weakness of the fey, and threatened to cut his fingers if he tried to run.
The man, in fact Aloysius Starkweather, interrogated him regarding the revenge the faeries took on his family years before. It was through him that Aloysius was finally able to confirm that his granddaughter, Adele, was replaced with the sickly human child Elizabeth Moore, like a changeling. Sallows soon got into a fight with Aloysius. Sallows, seemingly aware of Mortmain's plans, taunted Aloysius that Tessa would be the Downworlders' vengeance against the Nephilim. It was left unsaid by Aloysius, as he told the story to Charlotte, whether or not he did cut off Sallows' fingers and slit his throat.
Sallows, being a satyr, had goat-like features. Aside from his trimmed beard and pointed jaw, Sallows had cloven hooves, neatly curling horns, rectangular pupils, and yellow eyes, half-hidden behind his spectacles.