Inferno is a standalone expansion to Arcadia Quest. It is 100% compatible with the original base game and introduces four new Guilds: Sharks, Tigers, Crows, and Serpents.
It features new types of heroes, such as Alchemists and Gladiators and a branching campaign system, in a storyline revolving around the guilds descending into a fiery abyss. There are Brimstone cards that make the terrain risky to navigate, and a new mechanism called "Damnation" that can corrupt the characters over time or change the behavior of nearby monsters.
There are also Angels, which are allied characters for the heroes to rescue, escort or assist. Working with the Angels can affect the branching campaign path system and even allow the player to recruit them for use in later missions.
Don't just take our word for it! Check out this review of Arcadia Quest Inferno.