The Humans fight back! And this time with Toa technology. At one time Brain Freeze was Sentinels twin, and much smaller. When the humans heard about entity's capable of killing Toa they immediately set out to find and capture one. After a long search they finally tracked down one, and after a long and horrifc battle, Brain Freeze was finally subdued.
He was then torn to pieces and then rebuilt from scratch, along with pieces of other Toa the humans created the Hardsuit. The hard suit has the ability to super-freeze itself and its surrounding environmen. A special coating on the metal, and insulation on the inside keep the suit from freezing, while any metal around it becomes brittle and easily broken. Brain Freeze is said to be the as powerful as Indica in close combat.
The bits of black you see on the chest are pieces of human alteration.