Once it has been socketed, a legendary rune inherits the property of any other upgrade of its type. You can save on using an Upgrade Extractor, too, since you’ll be able to freely socket and remove these runes from any type of armor.
Additionally, these legendary runes will provide the same set bonuses as any other rune you may possess. You’ll be able to combine these with exotic superior runes, “For example, if you have a Superior Rune of the Adventurer in your helm, and you add a legendary rune to your boots, setting the legendary rune’s suffix to Adventurer will grant the two-piece bonus for that set.”
If you wish to obtain these legendary runes, you’ll need to use the Mystic Forge and provide gifts, including a gift of new materials unlocked by global events.