Red steel

Red steel is a substance that originates in the Savage Coast on the world of Mystara.

To avoid the deforming effects of the Red Curse, a mutagenic plague that afflicts the people of that region, a person must wear cinnabryl, a rare magical ore. As its magical energy depletes with time, cinnabryl becomes red steel. This lightweight yet durable metal can be forged into weapons capable of striking even magical creatures not wounded by normal steel. Naturally, then, cinnabryl is coveted, its acquisition motivating individuals to adventure and nations to war.

Factol+Sarin wore armor forged from red steel.

Red steel should not be confused with Abyssal bloodiron.

Sources:

Savage Coast Campaign Book by Tim Beach and Bruce Heard, from WotC's Old Editions Downloads page
The Factol's Manifesto