Bane of Immortals is Mythical with a total supply of 143
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Only after decades of devotion, mastering all the specific disciplines and completing all your assigned tasks does one become a master at the Arcane university.
This accomplishment is, amongst other things, rewarded with a ring called the 'Bane of Immortals' which has very destructive power.
So when the neophyte came running in and spoke to the gathered masters about a portal that had opened not far from the university with otherworldly beings pouring out of it, the masters did not shrink back in fear.
Instead, the masters ran quickly from the hall to the location of the portal, and confronted the beings.
Activating their rings, a pink circle of mystical energy spiralled at the masters feet, expelling a blast of heat that scorched their attackers.
With the power of the rings being too great, the beings retreated back into the portal from whence they came!