After the collection of Dr. Dark Gulian Volume 14, the strongest weapon material "Semlya Stone" was exchanged in the last chapter "imelda", an antique shop in Backstreet. And build the strongest weapon on the first floor when trapped in the IBC building in the later period. Note that if you want to be a "bibliophile", you must buy the eighth issue of crosby Times, and then exchange novels for props.