Tamahagane (珠鋼?) is a special type of steel with divine properties. Purifying tamahagane creates impurities called noro, which can agglomerate and turn into destructive lifeforms called Aradama when left unchecked. On the other hand, purified tamahagane can be refined and further processed to craft swords called okatana, the weapon Toji use to slay Aradama.
Tamahagane is actually a real-life type of steel being made through special process by Japanese sword blacksmiths to create Japanese melee weapons like swords, knives and other tools.