The Master Builder, born Faber-of-Will-and-Might, was the head of the Forerunner Builder rate, and de-facto leader of all of the Forerunners during the later years of the Forerunner civilization. He was a cunning politician, and fought fiercely for power with the Didact following the Human-Forerunner Wars and up until the Didact’s exile on Erde-Tyrene. Both Forerunners fought over the best plan for combating the Flood should they return, with the Didact a proponent of building shield worlds, and the Master Builder the head figure in favor of building the Halo Array. While the Didact was able to use his popularity after winning against ancient humanity for many centuries after, the Master Builder eventually won out, and forced the Didact into hiding for millennia afterwards. During that time, the Master Builder further consolidated power with his rate, commissioning the Halo Array, which was designed and built by his own Builders.
Prior to the exile of the Didact, both he and the Master Builder worked together to design and build Mendicant Bias, the most powerful ancilla ever built by Forerunners, who was put in charge of the defense against the Flood. Once the Didact was exiled and the Halo Array built, the Master Builder sent Mendicant Bias along with Installation 07 to Charum Hakkor to perform a test of the halo system. During that test, the Primordial was released, and Mendicant Bias was eventually corrupted by the logic plague.
Following the return of the Didact, the Master Builder used his power to capture and imprison both the Didact and Bornstellar Makes Eternal Lasting, sending Bornstellar back home, and the Didact into a Burn, an area overrun by the Flood. This proved to be one of the Master Builder’s last actions as head of the Forerunners, however, as he was soon after charged with crimes against the Mantle by the Forerunner ecumene.
During his trial on Maethrillian, Mendicant Bias and the Primordial attacked the Forerunner capital, killing almost everyone on and around the capital world, though both Bornstellar and the Master Builder survived. After the attack, the Master Builder no longer wielded the power he once did, but his leadership was still relied upon once the Flood had conquered most of Forerunner space. He was placed in command during the Battle of the Greater Ark, and though all remaining Forerunner forces including one remaining Senescent Halo were used in the battle, the Forerunners lost and the Master Builder was killed.