Quintus Cassius is a human male, which Harry Dresden nicknames Snakeboy and Liver Spots. He is a member of the Order of the Blackened Denarius, and host of the Fallen Angel Saluriel. He first appears in Death Masks.
He seems to have some magical talent. He conjured snakes and flung them onto Susan Rodriguez. He aimed a curse at Harry Dresden and Susan Rodriguez outside Dresden's apartment after Susan helped Shiro Yoshimo rescue Dresden from Nicodemus Archleone.
In human form: as Father Vincent, he was thin and of average height, tanned, between forty and sixty with greying and neatly cut hair. He wore a black suit and white clerical collar with a rosary and crucifix.
As Snakeboy, he is covered in dark gray scales with hints of red rust. Looking more or less human from neck to waist. From neck up and waist down, he looked like a flattened serpent. He had only coils for legs that slithered behind him. He had two sets of eyes; one gold and serpentine, the other glowed blue-green inside the first pair. He had a sigil of pulsing light in the scales of the snakes head.
After losing the coin, Cassius was so grey, he blended in with the zombies, but he has spots of color on his cheeks. He was of "medium height," he had long thinning hair and loose skin with a lot of liver spots and he moved like someone with arthritis. Dresden had trouble recognizing him because he had aged rapidly, thus earning the nickname Liver Spots.
In the series
In Death Masks, Harry Dresden first meets him when he, Nicodemus, and Deirdre Archleone come to steal the Shroud of Turin at an undercover sale during a gala at the Marriott. He conjures and throws a bunch of snakes at Susan Rodriguez, they wrapped around her, striking.
He was impersonating Father Vincent. When cornered by two Knights of the Cross and Harry Dresden, he voluntarily gave up his Denarian coin in an insincere show of remorse, thus earning a measure of clemency in the eyes of the Knights. After the Michael Carpenter and Sanya left the room, Dresden broke his arms and knees with a baseball bat to get information about where to find Nicodemus Archleone and the Shroud—and ensuring that he was taken out of the picture for the duration of the events in Death Masks.
In Dead Beat, Cassius entered Waldo Butters' examining room at the Forensic Institute with Grevane acting as the "drummer" to keep the zombies under command. Cassius chased Dresden and Waldo Butters in a Caddy when they were bring to escape them at the Forensic Institute.
"Liver Spots" was with Gravane when they attacked Harry Dresden's apartment with zombies in an attempt to capture Waldo Butters. Dresden makes a deal to exchange the numbers locating the Word of Kemmler. They will drive off to safety taking Butters and Liver Spots then release Spots giving him the numbers. In the process of making the exchange, Corpsetaker attacked Grevane's zombies with an army of specters, enabling Dresden, Butters and Thomas Raith to escape.
At the Field Museum of Natural History, Cassius captured and tortured Harry Dresden for revenge and in an attempt to obtain Lasciel's coin from him. The torture was interrupted by Waldo Butters and Mouse—Mouse snapped his neck on Dresden's command. Before dying, he cast his death curse "Die alone" on Dresden.