Aegizatylus

The Aegizatylus (, Eijizatairausu; lit. "Impregnable Castle Wall of the Aristocratic Domain") is a unique form of Caster Magic that's used since ancient times, acting as a form of defense of many territories and settlements; it's also one of the myriad forms of Subspecies Magic in existence, obviously derived from the mechanics of Barrier Magic, taken to such extremes to ensure its utmost effectiveness. It's power easily surpasses nearly all forms of Lost Magic currently known, as it utilizes a raw power that magnified through unknown methods — differing between its respective users.

Considered to be the "ultimate defense", something that's proven true on certain occasions throughout history, it's said to be a personification of — the ruler using their immense powers to protect those under their dominion. Typically classified as an Ancient Spell, it's been passed down from leader-to-leader and commonly used in the modern era by various government officials, nobles, tribe leaders, and all forms of royalty. Its epithet is the "Unbreakable Shield of the Gods" (神の不滅盾, Kami no Fumetsu Tate), alluding to people's statements of being protected by a divine power, placing their complete faith in its capabilities.

Trivia

 * This is based on the Wall, an impregnable barrier which guards the city of Insomnia, in the game, Final Fantasy XV. Like the aforementioned, the article shares similar effects but retconned as to allow different methods of casting without it steadily killing the caster. Considering the various dangers present in the Heroic Chronicles storyline, the article is rather needed :P