"This once-fearsome sword has seen better days. It can do some damage in the right hands but also breaks quickly."
The Rusty Broadsword is a melee weapon found in Breath of the Wild. It has a base power of 6, and can be found in lone locations around the majority of Hyrule. It appears to have once been a Knight's Broadsword, but has experienced severe damage, as the majority of the blade is rusted.
These items are found laying around fairly abundantly in early parts of the game. One can be found on top of a small island in the pond, just outside the Shrine of Resurrection on the way to the Temple of Time. They are often found in the various ruins of Hyrule.
With a durability of just 8, it is the most fragile of swords that Link can acquire, with it breaking just as quickly as a Torch or Boko Club. With an attack power of 6, it is more powerful than the Traveler's Sword and makes for an okay weapon during the early portions of the game, but it quickly becomes obsolete as Link acquires more powerful weaponry.
Like all the of the other Rusty weapons that Link can acquire, he can toss it in front of a Rock Octorok. When the Rock Octorok inhales the sword, he will chew it up and spit it out back towards Link. This will upgrade the weapon and it can be spit out as a Traveler's Sword, Soldier's Broadsword, Knight's Broadsword, or even a Royal Broadsword.
Hyrule Compendium entry of the Rusty Broadsword.