From Wikipedia- http://en.wikipedia.org/wiki/Painkiller_(video_game
"The game is inspired by such classic titles as Quake and Doom, with the emphasis on killing large numbers of monsters. The game is divided into five chapters, each about five levels long. The player's objective is to get through each level, from start to finish, by slaughtering hundreds of monsters. One of the game's most important aspects is its diversity. Each level presents a new location with various themes and art styles. The levels include castles, monasteries, an opera house, Venice, graveyards, and more. Monsters are also very different, with new ones almost every level. There are five "boss" levels where the player fights a boss. The game presents an option to complete a special task each level, which results in the player getting a "tarot card", a rare, hard-to-obtain bonus. Each equipped card provides different bonuses for a small amount of time, like powered-up weapons or slow motion."