There is a hidden message after the Perseus boss battle. Stepping on the three white stars with Kratos' left foot and heading back to the tree directly behind the Reins of Chaos will cause Kratos to start absorbing red orbs from the tree. As this happens, a message will be written in the sky.

The message reads "Now I am become death, the destroyer of worlds." This is a line taken from the Bhagavad Gita, a piece of Hindu scripture famously quoted by J. Robert Oppenheimer after witnessing the detonation of the first nuclear weapon in 1945.
Contributed by DrakeVagabond