Atriox's origin story has several key elements. First, his origin within the Covenant as a Brute, a species that was not treated equally. Second, his awareness of the unfair power structure where the Prophets held all the reins. Third, his courage to take action against this established order. Fourth, his ability to attract followers, whether they were fellow Brutes or others who also wanted change. These elements combined to create the story of his rise from a disgruntled member of the Covenant to a powerful leader in his own right.
Well, Atriox's origin story begins with his dissatisfaction as a Brute in the Covenant. He saw the unfair treatment his kind received from the Prophets. This led him to form his own army. He was a natural leader, able to inspire loyalty among other Brutes and even some members of other species. His origin story is really about his fight for equality and power within a very unequal and hierarchical system like the Covenant.
Atriox is a powerful Brute leader. In the Halo Wars 2 Atriox Story, his story is about his rise to power and his conflict with the UNSC. He has a strong will and great combat skills, leading his forces against the humans in a battle for control of the Ark.
There may also be other Brute subordinates of Atriox. They play important roles in following Atriox's orders and contributing to the battles against the UNSC. For example, they might be in charge of different units in the Brute army and help Atriox execute his military strategies.
Michonne does die in the comics. The circumstances surrounding her death were quite dramatic and had a significant impact on the story's progression. It happened during a major conflict with other characters.