The Mithril Door is located on the upper level of the Ancient Cavern with the Mithril dragons. The door can be opened with the Dragonkin key - obtained as two pieces (strange key loop and strange key teeth) during the While Guthix Sleeps quest, which can be fixed together by using one of them on a mithril dragon.
Within the mithril door is a large orb, and three hanging dragon heads. Casting Fire wave or Fire surge on each of the dragon heads light them, and once all three are lit they light the large orb. This then causes the Dragon Forge to activate, so the dragon platebody can be made there, providing the player has the ruined dragon armour lump, ruined dragon armour shard, ruined dragon armour slice and a blast fusion hammer on them, as well has having level 92 smithing (or is being assisted by a player who has 92 smithing).