The First Blade was an ancient weapon that served as the foundation for all modern lightsabers. It was designed by an ancient Je'daii Master known as the Weapon Master and kept on Tython by droids for millennia.
The First Blade was originally crafted by a Je'daii called the Weapon Master on Tython. During the Force Wars, Jedi High Council member Rajivari searched to retrieve the First Blade. He proved worthy of it, and was permitted to take the hilt. Rajivari heavily engraved the hilt to store information about the location of his secret knowledge reservoir, and then he returned it to its droid keepers.
The hilt of the First Blade was recovered by the Scorekeeper's Herald at the time of the Cold War. This Jedi then mounted the hilt into a lightsaber, complete with a crystal that had been found in the Tythonian Forge. Yuon Par, the Herald's Jedi Master, also thoroughly studied the engravings on the hilt to deduce the coordinates of the Fount of Rajivari and let her apprentice reach it.
The First Blade was introduced in the 2011 game Star Wars: The Old Republic as part of a class-quest for the Jedi Consular. How it fits in history with the protosaber is currently unknown, but the hilt of the First Blade has no cable or power pack whatsoever. However it is possible that it was modified to work as a modern lightsaber due to the conflict in that era. Combined with the knowledge of Forcesabers used by the Force Hounds of the Infinite Empire, it may have been descended from Xesh's weapon, or it may have been necessary to have balance in the Force to use it.