The dragonkin key is created by engaging a mithril dragon in combat and subsequently using, in any order, both the strange key teeth and strange key loop on it. In order for this to work, you must deal damage to the dragon. The two strange key parts are found in Movario's base. The completed dragonkin key permanently unlocks the mithril door, located west of the stairs leading to the lower level of the Ancient Cavern, and is consumed in the process.
Inside the room, players may activate the dragon forge by activating each of the three dragon heads. In the dragon forge, players are allowed to make the dragon platebody if they have all three ruined dragon pieces and 92 Smithing. The mithril dragon used to create the key will temporarily appear to be level 105. Fusing the key will cause the dragon used to become unaggressive and unattackable.
This information has been compiled as part of the update history project. Some updates may not be included - see here for how to help out!