In a nutshell, LGS involves collecting various ingredients, crafting weapons of “nearly any kind” or accessory/equipment slots, and upgrade the items several times to make them stronger. Ingredients may be found in “all three raids,” Turbine says, and it plans for tier one and tier two materials to be found in various quests. The full post features a lot more info, if’n you’re interested in DDO crafting.
Turbine is adding legendary green steel crafting to Dungeons & Dragons Online. It’s a “wide-reaching, complex crafting system,” according to an equally wide-reaching forum post. The wall o’ text lays out the mechanics at a high level and also explains how legendary green steel crafting differs from heroic.