Thanks!
Couldn't have said it better myself.
And rest assured, you will be rewarded for your efforts after Paldemar has been thwarted.
And that random door towards the back of the trading post wasn't a normal door. It was the duergar brothers' personal escape tunnel to their 'home' which would have certainly swallowed you all whole and spat out your bones. In metagame: it led to the underdark. None of you were ready and I didn't want to waste your time by giving you encounters that you would be forced to run from.
But I think I should take this opportunity to give some more background on some of the people and places you've encountered:
Seven Pillared Hall - the sole settlement under the Thunderspire. Hundreds of years ago, minotaurs founded the city of Saruun Khel in the depths below the Thunderspire mountain. Saruun Khel soon became the center of an oppressive minotaur kingdom that subjugated neighboring lands. However, three centuries ago, a battle for the throne of Saruun Khel erupted into a vicious civil war. Out of anger at the minotaurs' growing devotion to the deity Torog, the demon lord Baphomet cursed them with mindless fury. Thus, when one faction was defeated, the survivors took to battling one another until only a few minotaurs remained in the ruins of the once great city. Today, the Seven Pillared Hall is a safe haven to those who wish to prosper from the treasures of the Underdark while skirting the dangers that await so far beneath the earth. The settlement is policed by Brugg, a burly ogre who bears the mantle of Sheriff. However, behind the scenes, the Hall is really run by the Mages of Saruun, a committee formed by mages who came to explore the ruins of Saruun Khel. They found many treasures, developed great power, and established the Seven Pillared Hall to provide safe access to the denizens of the Underdark.
Grimmerzhuul duergars - a family of duergar who have prospered in the Hall from their mining ventures in the Underdark. However, the citizens of the Hall don't know that the Grimmerzhuul are also enjoying the benefits of a booming slave trade. Headed by Murklemor, the duergar deal in kidnapping less prominent members of the Seven Pillared Hall and selling them to anyone who would have use for them. As of late, the grimmerzhuul have had many deals trading their stock to the Blackfang Gnolls.
Horned Hold - residence of the Grimmerzhuul clan. Once a great fortress of the minotaur army, the duergar now use this place to conduct their less than desirable business as well as defend themselves from 'competitors.'
Blackfang Gnolls - a clan of gnolls loyal to Maldrick Scarmaker and the demon lord Yeenoghu. These gnolls are known for their ferocity, blood lust, and association with the demon lords.
Maldrick Scarmaker - leader of the Blackfang Gnolls. Maldrick is a devout cleric to the demon lord Yeenoghu, Prince of the Demons and Lord of the Gnolls. He is the bestial embodiment of savagery and butchery and expects his kin to follow in kind.
Well of Demons - a once thriving temple to the Demon lord, Baphomet, it used to serve as a proving grounds for the Demon's minotaur followers to ensure that only the strongest served the horned demon. Today, the place is inhabited by Maldrick Scarmaker and his clan of Blackfang Gnolls. Maldrick seeks to usurp the powers hidden in the temple and divert them, through the use of some religious ritual, to his lord, Yeenoghu.
Baphomet - Prince of Beasts who holds dominion over a layer of the Abyss called the Endless Maze. He is the self proclaimed Lord of Minotaurs and the embodiment of rage and vengeance. Also known as the Horned Lord and The Trampler.
Paldemar - part of the Mages of Saruun, a council of mages that overlook the Seven Pillared Hall and most everything else in the Thunderspire Labyrinth. You've gathered enough evidence to know that Paldemar is behind a major plot involving the Blood Reavers, the Grimmerzhuul duergar family, and the Blackfang gnolls. You can only guess as to what his plans truly are, but you can tell that you are close to finding out.
If you have any more questions you would like answering, I'd be more than happy to answer, so long as it doesn't give any spoilers away.