This cyborg unit was originally a security bot, assigned to protect a construction site. But that site was one of the first ones hit when the bombs dropped.
The unit was very heavily damaged. But due to its self-repair protocols it pieced itself back together using parts and tech salvaged from the destroyed construction site. It became sort of a Frankenstein's monster, with parts of all different types of heavy machinery melding together into one powerful body. Hence the tag of being a "mutant". It turns out that after this mutation, it is far more dangerous than its original form.
Without a site or any workers to protect, it's operating aimlessly. It's not a fully sentient organism as it relied very heavily on a prime directive (site security) in order to operate. Those directives are now void, so left to it's own devices it's become an unpredictable and violent to anything it deems a threat (even when something clearly isn't a threat). Now it's just wandering the wastelands, and shredding whatever living things that cross its path, especially humans. It's legend as a harbinger of death has spread across most of the human settlements, and it's basically the most feared thing in their lives. The name Warmask was given to it by those who told the first tales of it's unspeakable horrors.