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.
|Dungeons & Dragons Campaign Settings|
|Basic D&D:||Mystara (Blackmoor) - Pelinore|
|AD&D:||Birthright - Council of Wyrms - Dark Sun - Dragonlance |
Forgotten Realms (Al-Qadim - The Horde - Icewind Dale - Kara-Tur - Maztica)
Greyhawk - Jakandor - Mystara (Hollow World - Red Steel - Savage Coast)
Planescape - Ravenloft (Masque of the Red Death) - Spelljammer
|3rd/3.5 Edition:|| Blackmoor - Dragonlance - Eberron - Forgotten Realms |
Ghostwalk - Greyhawk (Sundered Empire) - Ravenloft
|4th Edition:||Blackmoor - Dark Sun - Eberron - Forgotten Realms - Nentir Vale|
|5th Edition:|| Dragonlance - Eberron - Forgotten Realms - Greyhawk |
Nentir Vale - Ravenloft - Ravnica - Spelljammer
|Third Party:|| Dragonmech (3E) - Dragonstar (3E) - Kingdoms of Kalamar (2E/3E/4E) |
Midnight (3E/4E) - Ptolus (3E) - Scarred Lands (3E/5E) - Spellslinger (3E)
Wilderlands of High Fantasy (Basic)