The Crown of Thorns is an Atlantean defensive network.
An ancient relic from a long bygone era in Atlantis, the artifact known as the crown of Thorns is a vast protective network of deep mystical nature entwined with greater Atlantean science which has served as the ultimate defense from outside invasion. The magic of such an apparatus is such that it can resist the force of crushing tidal waves and even an assault by some of the most powerful individuals the world has ever seen, the influx of magic from the Crown also has the effect of accelerating Atlantean adaptation to undersea life. Causing those who sit beneath its embrace to take more after the aquatic life who live below the sea, hence the sudden surge of sea changed citizens springing up in the lower Trides of the city. The only way to enter and exit the city is through magical medallions for easy recognition by the barrier, but its effects aren't without fault. If the Amulet needed to enter the Crown of Thorns is damaged it could lead to disastrous after effects for the wearer.
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