Metal Devouring Insects

They were an extremely unique kind of Magical Beast. When they were fully mature, they would still be only the size of a fist. They didn't have a crystal core, but they had been classified as third rank Magical Beasts.
That was because a single matured Metal Devouring Insect could kill a third rank Magical Beast. The most terrifying thing about them was their teeth.
They relied on eating metallic ores to live. Not only were their bodies hard as metal, but they were also extremely tough. External force attacks were mostly unable to kill them.
The most terrifying thing was that Metal Devouring Insects were Magical Beasts that lived in groups. It was common for hundreds or even thousands of them to gather together and dominate a vein of ore. Only when that vein was exhausted would they move on together to find another vein.
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