Legendary Items include Legendary weapons and armor sets in Greedfall. These weapons and armor are the best and most powerful of the game. They deal high damage and provide great protection but as expected, are tough to get.
Legendary weapons and legendary armor sets in Greedfall are to either found in a hidden chest or after you have fought a boss. You might get a key for these hidden chests and then you can open them.
GreedFall Yataghan Location
When you reach Glendgnamvar The Shore of the Tall Bones, you will enter the Ancient Secrets mission where you will come across a boss that you ave to defeat in order to get the weapon.
After you defeat the boss, you can grab the weapon from the chest near him. If you aren’t on a higher level and it is impossible to kill the boss then you can even try to sneak in and grab it, though it might take a few tries.