The Inner Sanctum and the attack on the Seven Pillared Hall

3, Hearth Fire 90AN
A mighty roar echoes throughout the chamber, and traps meant to thwart those who mean to enter the inner sanctum spring to life. The adventurers are spread throughout the central chamber. Cross bow turrets, chained undead, magical statues, an elemental vortex and a doom sphere made of dark energy, however, are not the least the party should fear, for the entrance to the inner sanctum is protected by The Guardian, a young adult green dragon who is bound to this wretched place. The party trembles as they behold this new threat, though their fears are quickly abated when Rasic charges headlong into the dragon, rending it with two critical blows. The rest of the party are able to regroup and together they bring down The Guardian and the final foe standing between them and the inner sanctum. 
As the iron doors to the inner sanctum open, the heroes behold a hall that is filled with a thick grey mist, through which no light or sound can pass. The mist represents a final test of bravery, which is quickly passed by Ivgaar and the rest of the party. Undeterred, the heroes quickly traverse the short hall and enter the inner sanctum of the Well of Demons...
Within the inner sanctum, the party find Maldrick Scarmaker, a foul creature that seems to be half Gnoll and half demon attempting to finish a malign ritual. Entranced in a magic circle are the two missing Riverdown civilians, and Maldrick is flanked by Barlgura, Carnage Demons and a massive Minotaur skeleton. As the fight begins, the two tiefling brothers suddenly turn on the party, and the adventurers find themselves beset on both sides by enemies. The heroes battle courageously, and though in the the end they are able to defeat Maldrick and his guardians, it is not before the demonic Gnoll finishes the ritual and the two Riverdown captives disappear with a scream and a flash of light: a sacrifice to the Demon Yeenoghu. The two brothers disappear into the mist and flee the inner sanctum as soon as it becomes clear that the party will be triumphant.
Maldrick dies laughing as a great shudder passes through the well and the cauldrons in the inner chamber start to spew black smoke. The massive statue of Baphomet that dominates the inner sanctum shatters, and in it's place stands a Statue of Yeenoghu. The Wizard and Artificer determine that the ritual was bringing the Well of Demons under the control of Yeenoghu, and that it can be reversed with the right ritual. On Maldrick's corpse, Ebben finds correspondance between the Gnoll and Paldemar, as well as a silver key. When Ebben takes possession of the key, he receives a vision that shows him a path close to the Seven Pillared Hall. The Wizard determines that they have 24 hours before the doors to the inner sanctum reseal themselves, so if the heroes wish to reverse the ritual, they must return within a day.
As the heroes rush back through the Well of Demons, massive spacial disruptions wrack the fabric of reality, and the well starts to shift and change, rearranging itself more to Yeenoghu's liking. The adventurers come across evidence of recent battles as they return the the hall, and find the gruesome remains of several miners who were apparently torn apart.
The party finds the door to the deep stair hammered open, and Seven Pillared Hall in Chaos. A veritable army of Gnolls, Hyenas and demons are waging war on the denizens of the hall. The Ordinator Arcanis, flanked by 6 bronze warders, summons massive serpents of ice and fire, laying waste to demons left and right. As the party attempts to return the the Halfmoon in to save their friend, Rendil, they happen across a Drow caravan that has been overrun with Gnolls, where a lone survivor is making his last stand. The party helps this lone Drow fend of the Gnolls and Hyenas, and this is how the heroes come to know Velkyn Locksinger of house Hunzrin. The group decides to form a temporary alliance in order to survive the assault on the hall.
As the adventurers and their new found ally make their way to the Halfmoon in, Viserys is struck with a vision, which the rest of his allies are pulled into. This vision shows the heroes their inevitable fall, as hundreds of bronze warders advancing on the Seven Pillared Hall, slaughtering everyone inside. The vision then shows the party an army of Bronze Warders, Gnolls and Demons marching on the Nentir Vale, overrunning Fallcrest, Hammerfast and Winterhaven. During this vision, Viserys is marked with idol of Ioun. When the vision ends, the party find that their bodies feel well rested, and they have regained use of expended spells and powers.  It is with this revelation that Eben leads the party into the warrens near the hall, to what appears to be a dead end.
Drawn by the silver key to a very particular spot, Eben places the key in what appears to be solid rock, which splits open to reveal a hidden chamber.  The chamber is empty, save for teleportation circle in the center of the floor. As the adventurers approach the circle, they are confronted by a skeletal apparition: A Manifestation of Vecna. This manifestation guards the entrance to the Tower of Mysteries,
The Manifestation of Vecna demands secrets, knowledge, lore and the adventurers souls. The Wizard, Artificer, Warlock and Cleric attempt to impress the Manifestation of Vecna, while Velkyn spins some convincing tales, but unfortunately, the apparition's hunger for lore is not satiated. He allows the adventurers entrance into the Tower of Mysteries, but at a cost. He robs the heroes of their ability to use one of their powers or exploits for the remainder of the day, as well as stealing a portion of their life force. After paying their toll, the party steps into the circle and are teleported to the Tower of Mysteries.