Kurotowa
Legend
Alright, spitballing from first principles here. In D&D 5e we already have mithril armor, which is extra light, and adamantine armor, which is extra hard. So we need a third quality to add here, preferably one that's both useful to adventurers and thematic with gold. And I am looking at armor, because weapon material isn't really a thing in 5e, it's more just flavor for how and why a weapon is magical.
One obvious idea is to make it heavy, the counterpart to mithril. But it's hard to theorize a useful armor that's super heavy. We could maybe make it magic conductive, but that's just a generic component for magic items. Well, how about magic resistant then? It wouldn't be too hard to steal the mechanics for Spellguard Shield where you have advantage on spell saves and spell attacks against you have disadvantage. A little unoriginal, but serviceable. Or heck, maybe just rewrite Spellguard Shields as Orichalcum Shields. Say that orichalcum is too rare and too heavy to make full armors out of it, but a shield is enough to ward of many harmful magics.
But to get back to the original request... if you want an orichalcum sword, just pick any magic sword in the book and give it a fancy look and history.
One obvious idea is to make it heavy, the counterpart to mithril. But it's hard to theorize a useful armor that's super heavy. We could maybe make it magic conductive, but that's just a generic component for magic items. Well, how about magic resistant then? It wouldn't be too hard to steal the mechanics for Spellguard Shield where you have advantage on spell saves and spell attacks against you have disadvantage. A little unoriginal, but serviceable. Or heck, maybe just rewrite Spellguard Shields as Orichalcum Shields. Say that orichalcum is too rare and too heavy to make full armors out of it, but a shield is enough to ward of many harmful magics.
But to get back to the original request... if you want an orichalcum sword, just pick any magic sword in the book and give it a fancy look and history.