Once just a normal Blank, Grand master found what remained of Tabuu and absorbed it, becoming a tall, lean version of a Blank, complete with Tabuu's wings. It then organized its fellow Blanks, claiming to be their "destined leader" who would take them to the "promised land," which happens to be the world our herous reside in.
Grand Master possesses few of Tabuu's fearsome attacks and instead relies on weapons and his allies to protect him. He did, however, inherit Tabuu's ultimate technique, the Off wave.
Ultimately, Grand master was defeated by the combined effort of all 60 heroes. Its armies were destroyed, and its ambitions were crushed all in this single assault.