The Weapon Master was an ancient Je'daii Master and a member of the Je'daii Order. They built the First Blade, which opened the way to lightsaber crafting.
After their death, the Weapon Master's ashes would remain hidden on Tython, along with the hilt of the First Blade. The resting place of the Master was rediscovered in the ruined temple-city of Kaleth millennia later by the third Barsen'thor.
The Weapon Master was first mentioned in the 2011 video game Star Wars: The Old Republic. Due to the nickname, this individual might have been referring to Tem Madog, the Je'daii Temple Master of Vur Tepe where the Je'daii crafted for the Order; including its weapons.