The Black Spartan is a hidden biped that is loaded inside the Autumn towards the end of the mission. It was used as a scaling biped, comparing the size of a regular spartan model to the Autumn. It's purely technical, and was never meant to be seen, let alone reached. 

Recently we discovered a new way to get to the Black Spartan by falling onto him from above using a suspended box through loadzone manipulation. You are about to witness its process and unfolding.

Enjoy! 

https://www.youtube.com/watch?v=DvbTyU6ehOA