Great Door

The Great Door was a massive door hidden on the outskirts of Waterdeep on the Sword Coast North. It served as a gate to the planes.

Defenses
The Great Door was constructed inside a huge pit in the ground, covered over with an illusion of solid ground.

After it was tampered with, the Door could shoot bolts of magic at those outside, and release monsters from other planes. It could also levitate into the air, tearing up the ground with it.

Description
The Great Door was a colossal pair of doors set into the cliff-face of the pit, bearing many human and monstrous faces and topped by a statue of a winged, dragon-like creature that looked down on those below.

Powers
Described as a dimensional doorway, The Great Door contained a gate that unlocked an interdimensional wormhole to the planes.

History
The Great Door was constructed by two great wizards, Ostus Agrivar and Parwyyd Hanifar, who worked together to explore the frontiers of magic. They built the Door as a means to easily enter the planes. Each wizard needed the other to control the Door, however, so it remained closed after Ostus's death in the Year of the Dragon, 1352 DR.

By the Year of Shadows, 1358 DR, the Great Door had been tampered with by an unknown power. It stood ajar and monsters from other planes emerged from it. Some people grew understandably rather angry about this, and believed that killing Parwyyd would cause the Door to close again. With a small military force, they laid siege to Parwyyd's tower, and he considered them "misinformed".

To deal with the Door, Parwyyd lured Kyriani Agrivar to the site. She was the "blood of Agrivar"—Ostus's daughter—and bore his magic aura, so she could take Ostus's place in controlling the Door. However, one of the faces on the Door spoke, taunting Parwyyd that he no longer had control, and flying kenkus emerged. In the fight, Onyx the Invincible briefly fell through the Door, and saw what looked to be the Hells. Dunstanny—Parwyyd's personal assistant—explained that Parwyyd and Kyriani needed to work together close the Door, but as they debated it shot a bolt of magic that separated the merged being Kyriani into Cybriana and Kilili.

The power that had tampered with the Great Door then seized greater control, raising it into the air and untethering the interdimensional wormhole from the Door, having it appear in the skies over Waterdeep instead. Kenkus and gargoyles swarmed over the city, and were engaged by Vajra Valmeyjar and Timoth Eyesbright, before the City Guard and Khelben "Blackstaff" Arunsun took over.

In the Hells, a gargantuan and monstrous creature, the one who waits, had seen the other side of the Door appear in its realm. It believed it to be a gift from the god Bane, Lord of Tyranny, and slithered toward it. Its attendants, the gargoyles and kenkus, had gone on ahead.

The interdimensional wormhole returned to the Great Door, and the kenkus and gargoyles emerged in the pit, followed by a fire elemental, while the branches of a nearby tree turned to serpents. Cybriana, Onyx, Parwyyd and Dunstanny battled the kenkus and gargoyles until Kyriani was ultimately restored. Working together, Parwyyd and Kyriani first let the door open wide to let the one who waits through, then used telekinesis to slam it shut on its trunk, slaying the monstrosity. Parwyyd then destroyed his Great Door with a disintegrate spell, and the threat it posed was ended.

Khelben later theorized to Kyriani that evil gods—Bane and his allies, most likely—had taken control of the Great Door, as part of some scheme leading into the events of the Time of Troubles that year.