The angel/demon cross Belial is a feared demon of Lucifer's army. Having now escaped one of the many prisons of Lucifer, he is out for revenge on those who framed him. As Belial, you must fight 40 types of enemies including huge bosses in purgatory - the land between heaven and hell. The levels feature horror-adjusted versions of familiar worldwide beings from mythology and history - gladiators, mummies and ninjas all become frightening foes.
Painkiller: Overdose is a raw first-person shooter similar to the previous Painkiller games; you run and gun for your life using a devastating arsenal of six different weapons. Originally developed as a fan-made modification, the team was given funding to make it into a full, independent game. The weapons have both primary and secondary firing modes. You may also combine these two and get a third firing mode as well.
Power-ups are again achieved via the Black Tarot cards, which set specific challenges and rewards for each level. The Demon Transform feature from the original also remains - it allows you to swallow the souls of your victims to use as charges for a smart-bomb effect.