What lies behind The Door That Waits?

The Door that Waits. The Dreadgate.   Deep beneath the Dwarven city of Mithlonde stands a heavily guarded door. A massive Blacksteel door no one has seen behind in millennia. No Dwarven citizen or traveler may enter neither the cave nor the descending staircase that leads to the door unless given explicit permission by each of the eleven clan leaders. What lies behind the door? When was the last time it was opened? Why is it so heavily guarded? Mothers warn their children of ancient evils that live behind the Dreadgate. Adventurers speculate a long forgotten palace of older Dwarven kingdoms lie in wait with treasures to be plundered. Most brush it off as simply a means to entering a secret stronghold for the elite in case of subsurface catastrophe, but only a handful of mortals know what truly lies behind the door that waits.   The Dreadgate   The gate is seemingly made from a combination of alloys and metals.   Varnokstrohl, or Blacksteel, is the main metal used in it's contruction. The metal is coarse and blackened. It hums faintly to the touch as if still ringing from the hammer that shaped the metal. It is highly resistant to corrosion and magical attacks. It has been hardened such that even extreme heats fail to weaken the metal of the door.   Drronzek is the next confirmed metal used in the Dreadgate. This common metal is used widely by Dwarven smiths due to its superior strength and stability once treated. Physically and magically resistant to attacks, this metal is used often in the creation of seals and doors. It is a dark silver color with subtle hints of sapphire blue ripples across its surface brought out during the shaping process. Only a handful of smiths are capable of producing 100% strength Drronzek as it requires immense deep pressure crucibles and coal gathered from far beneath the surface.   Aurgron Alloy is a golden bronze metal used primarily for locks, mechanical systems and inlays. Honey colored with faint grain patterns, the alloy is highly conductive to harmonic magic, and responds to sound based mechanisms like Clerical Tuning Keys or Dwarven chants. It is the primary alloy for the construction of the internal locks of the Dreadgate.   The Door that Waits is actually a set of two doors that stand over thirty feet tall with each door being twelve feet wide. From afar the gate has a dull matte dark silver and black tone with faint blue streaks when light catches it. Runes are carved shallow across the surface. Historical events mixed with a small amount of defacement. These runes have no baring on the mechanisms inside. Near the base of the gate, Aurgron alloy is woven stylistically to appear as roots or veins. They glow faintly a honey gold color and vibrate slightly to the touch.   The gate has no visible handles or hinges. Instead the locking mechanisms are hidden inside and between the doors of the gate. Twelve latches are deep within, only unlockable via a complex harmonic ritual. The "keys" for this system are three Dwarven tuning keys, each with their own role to play in the ritual. These keys are passed down from Cleric to Cleric within a rotation of the clans that reside in Mithlonde. No person knows all three key locations.   The cavern the Dreadgate occupies is, itself, a warning. An immense and domed cavern with a ceiling covered in stalagmites, and a polished stone floor. No natural light reaches this place, only lit by a few everlasting flames. The air of the cavern is heavy and cold. A dull humming can be heard throughout, its source: the gate. Guards are assigned to the top of the stairway that leads down to this gate, but they routinely check to make sure no unwanted guests have tried their luck at opening the door. Attempts have been made to dig around the gate, from above, or from below, all of which are met with the same unfortunate failure. The locks within the Dreadgate also create a physically impassable barrier emanating from the doors themselves so that no one can bypass the locks.