The Sun

The Sun is a spherical light bulb that provides light and energy to the majority of the World.

Endings
Return the Sun: 

Niko places The Sun at the top of the tower. It's brightness will intensify to the level necessary required to act as an actual sun for the world.

Return Home:

The sun is shattered. This will instantly end the world (see below) and return Niko home.

Niko
The Sun is the messiah's tie to the world. As long as the sun is unbroken, Niko remains trapped in the world of Oneshot. If the sun is shattered, the world will end instantly, but Niko can return home.

Function
Even if it is not at the top of the Tower, its general purpose still works as proven multiple times by Niko in the game. It can power the solar battery found in the barrens in an instant, and proves to be a functional source of light.