Red Steel forges into weapons of great power — Power that strikes fear into the most magical of adversaries. It is found only in the Savage Coast, where nations and races war among themselves, desperately trying to find it. With the search comes a curse — the Red Curse. It twists bodies into hideous parodies of their former selves. Only Cinnabryl provides protection form this Curse. But this protection fades with time, losing its magic. As it loses its protective veil, Cinnabryl slowly transforms into the powerful Red Steel. Survival and power in the Savage Coast are determined by the control of Red Steel.
Second edition AD&D is often fondly recalled for its weird and imaginitive campaign settings, but those were a curse more than a blessing: even at their best they relied too much on gimmicks and trickery that may have looked good on paper but provided little to no emergent gameplay, and at their worst they split the playerbase into tiny fragmented factions and ensured that none of the setting books sold very well - after all, why would you buy Ravenloft when your players are already doing Dark Sun? When do you find the time for The Old Republic when you're already heavily invested in World of Warcraft?
Red Steel is one of the less remembered, and indeed one of the worst examples of the above. The titular substance, the red tint it paints the entire setting in, and the resulting magic and mutation, are all rather poorly realized and are not integrated with the rest of the setting very well at all. The harsh deserts and magical drain of Dark Sun, the mists and horrors of Ravenloft, and the philosophy and alignments of Planescape, all sat at the very core of their respective settings, but in Red Steel the stuff's just... kind of there. It provides a surface layer of red stuff and mutation, but under that surface the rest of the lands and nations and communities and people are much the same as they would be without it. It's also extremely wordy, spending great many rather dry and uninteresting pages in explanation of all this stuff and the new rules.
It's not a complete waste, though. If you scrape past the initial red layer of the setting, you'll find a decently imaginitive and savage land, with a sort of colonial/mesoamerican flavour rather than your usual Medieval Europe. Its little nations and kingdoms, savage baronies and tribes and secret magogracies, are all brought up decently well into flavourful and distinctive settings for adventure. There's plenty of war going on, ancient ruins and dungeons to explore, and empty land for the higher-level players to carve for themselves. But even then it lacks in personalities and motivations, politics and intrigue: the powderkeg, that which the player characters are supposed to be the spark for, is sadly missing.
Had it spent fewer pages trying so hard to be original and flavourful and gimmicky, relegated the Red Steel stuff into a background element, and focused more on the characters and the nations, it could have been a real nice supplement. Even as it is, if you're running a game set in Mystara and your players are going off the beaten path, or you're looking to establish a colony land in your own setting, you could pull this one out and get a fairly decent starting point. But even then you'll have to work hard for it. Otherwise, you can probably safely give it a skip.