The devil slayer, diabolist, hellblade, and inquisitor (Dvl, Dbl, Hel, and Inq) are standard prestige classes as described in the DMG. All four are designed to give mortals a fighting chance against the forces of Hell.
The devil slayer devotes his life to combating the forces of Hell. Unlike the sneaky hellblade or bookish diabolist, he learns special techniques to defeat devils in single combat. Some churches of good train these warriors as an elite militant branch, while other devil slayers come by their training after being wronged by the creatures, out of sheer vengefulness.
Barbarians, fighters, monks, paladins, and rangers make excellent devil slayers, while clerics and druids add a spellcasting emphasis to the class. Rogues and arcane spellcasters rarely have the bloody-mindedness to follow this path.