Pyxis Prima is the box that contains the layers of history that the Firstborn has been imprisoned in. It in total has six layers including the original chamber.
The six layers are this, Present Day- Al Khali, World War II, The Crusades, Ancient Rome, Sumeria and The Original Chamber where the Firstborn was imprisoned. Each level has a boss, with Sumeria having more than one, as the last one you never have to fight. Al Khali's boss also you do not have to fight, as Arnold Leach only ever appears in cutscenes. The World War II boss is the psychic Nazi officer Lichthammer. The Crusades boss is Malteus, and Ancient Rome's boss is Senator Cassus Vicus.