Giant caddis fly larvae that builds its tube-like shell from metal and lives long in underground rivers. The only metal that won't eventually corrode in these conditions is either gold or magical steel.
The shell of the Castellian Caddis Fly is made up of shields, swords, spears and other weapons of legend. All washed clean of markings by the rivers flow. A shell of blades. Each one was the legendary blade of a particular culture or hero-cycle, made to kill gods and defeat evil. But eventually forgotten and lost. The shell is more dangerous than the fly inside it. Heaves itself up out of the white water, plumes of foam spilling on its spiked and shining sides.
When the fly needs new stuff for its shell. It finds the river system below a climax culture and waits a few thousand years for it to decline. Once the museums have burned and the hero's are dead, the blades will be forgotten and lost. Eventually they will end up in the water sink and the fly will recover them.