He has a habit of holding grudges. I had an associate who crossed him. It took years, but Dusk finally managed to track him down and the death he provided was not a quick one.
— China Sorrows on Dusk.
Dusk is a vampire who formerly worked for Baron Vengeous during the war but abandoned him after his defeat and capture by Skulduggery Pleasant. He would later return to his services upon his escape from jail and was tasked with finding and capturing Valkyrie Cain for usage in the Baron's attempt to return the Faceless Ones to our world. It was during this pursuit that he was sliced across the face with Billy-Ray Sanguine's straight razor, the wounds from which never heal, which caused him to swear revenge on her with his plan to turn her into a vampire and set her loose upon her own parents. However his plan was stopped by Spring-Heeled Jack and he was arrested and taken into custody in a Russian jail.
He was eventually broken out by Sanguine who enlisted his help in his own scheme to bring down the Irish sanctuary where he acted as the brute muscle as well as providing infected to be used to boost numbers. However this was all part of his plan to get revenge on Valkyrie and Jack and after biting the former he abandoned Jack to the mercy of Ghastly and Anton while he made his escape.
When it came down to it what really helped Dio to snatch victory here was that he was able to dish out and take more damage than Dusk could due to his greater strength and high regeneration ability. Dusk's unwavering belief in the Vampire Code also prevented him from killing Dio which made it harder for him to actually end the battle as Dio could power through all non-lethal damage.