The FFXIV Baron Helm is a Level 1 helmet that can be equipped by all classes. It’s typically used as a Glamour, as its stats are minimal, like most other Level 1 gear of its sort.
Players cannot acquire the Baron Helm in-game by normal means. Nor is it available as an “Optional Item,” like most other paid cosmetics. It’s only available with the Final Fantasy XIV: Heavensward Collector’s Edition. If you didn’t previously purchase the Collector’s Edition, you can upgrade to that version of the expansion using the instructions at this link.
As the service upgrade indicates, the Baron Helm is a reference to Final Fantasy IV protagonist Cecil — who begins the game as a soldier from the kingdom of Baron. The helm specifically references his original Dark Knight form.
It’s also important to note that, while the Baron Helm can be worn by any class at any level, it “will not be displayed when equipped by Viera characters.” At least not for the time being. This is because most full-sized helmets are currently incompatible with the Viera character model.