The Hand of Flame was an ancient Klingon military unit that existed during the Mythic Age when the tyrant Molor ruled the lands. They were a rebel group who opposed him and were led by three leaders; namely O'gat, Kollus and To'Kar. These leaders were each a master of a style of combat and they alone fought against fifty of Molor's warriors and were victorious in battle.
It is said that when Kahless the Unforgettable himself learnt of the rebel band's leaders prowess, he recruited them into his army. They then taught Kahless's army how to fight using their three different fighting styles. After Molor's defeat, they founded the Schools of Great Warriors with Kahless's blessing. (TNG video game: Klingon Honor Guard)