The Mithril door is located on the upper level of the Ancient Cavern with 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. Each head can be lit by left-clicking, and once all three are lit they move forward and 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).
- You can see the mithril door from the Vault of War using the orb of oculus. The reason the dragon heads are not there is because they count as NPCs.
- The mithril door is strong enough to withstand fishing explosives, as proven by Derril.