Boss is the name of the final stage where the player fights the Boss, a huge creature with it's maxilla facing down on you, and it's mandible (which also contains it's eye) facing upwards. While falling through the level, there are areas that have similar appearances and attributes to worlds 1, 2, 3 and 4 respectively, such as water during the world 3 area and less solid ground in the world 4 section. It also shoots a triple like blast at the player, before going through the next mini area. In hard mode It shoots two blasts.
The Boss also summons enemies from the respective area then when finally defeated shows the credits and unlocks Hard Mode