Red Steel is an Advanced Dungeons & Dragons "mini-setting", in that it is a campaign setting that focuses on a specific region of a larger campaign setting, ala Maztica or Al-Qadim focus on the South American and Arabian regions of the Forgotten Realms. Red Steel focuses on the Savage Coast region of Mystara, where conflict is driven over the titular magical mineral, which poisons those who wield it and turns them into mystically super-powered mutants.
Perhaps one of the reasons why it isn't well remembered is because the new races introduced in this setting were pretty much all of the furry mold; Rakasta (cat-people), Lupins (faux-French swashbuckling wolf-people), Aranea (shapeshifting spiders) and Tortles (humanoid turtles).
An updated version of Red Steel was released as "The Savage Coast" as part of the Odyssey lineup of D&D products. It also got some companion books focusing on the neighboring Orc's Head Penninsula.