The Well of Demons (as DM) was one of my favorite and most memorable of all the encounters in Thunderspire Laybrinth. In fact, the players went there TWICE.
I'll explain.
Their first mission to the Well is to find Sister Linora who has been sold to the gnolls as a sacrifice to Yeenoghu. Linora is a friend of some of the PCs and they feel very obliged to rescue her (after all, she is from Winterhaven and has helped them in the past).
The first encounter with the tentacle monster and the chokers went well. I described the thing as buckling under the flagstone like a wave, and then spiked tentacles flailing out at everyone. The PCs pushed through to the hyena dens and kept fighting more gnolls and eventually the gorilla-demon that shows up. This was challenging but not too bad. I think we had five 6th level PCs.
They find the Book that will open the way to the Inner Sanctum, but they don't know what it is yet.
Then they meet the three ghosts.
Personally, i think WotC skill challenges really suck so i mostly glazed over this encounter and roleplayed it without rolling dice. The cool stuff was coming up next and i didn't want to waste time with a dumb SC.
The Mirror Room proves to be very scary but nothing terrible happens in the end, except for a gnoll trapped in the oubilette who is briefly released and then sent right back

.
Before they enter the Blood Room they find a sixth party member to join them, so now there are six 6th level PCs (it was a minotaur warrior in stasis). As for the blood room, it was devastating. One PC died, another nearly died, and others were bloodied almost the entire encounter. They weren't sure of what to do at first and by the time they understood all the traps were sprung. The blood demons were tenacious bastards but it was ultimately the poison blood that killed the warlord, he drowned in it while unconscious.
So the PCs are beat up pretty bad, but they're under a deadline to rescue Sister Linora! She is going to be sacrificed and the Well of Demons will be rededicated from Baphomet to Yeenoghu who will become the new ruler (a fat imp of Demogorgon told them all of this immediately after the Blood Room).
By now they have found three of the items needed to enter the Inner Sanctum and lack only the Bell...which is in the Pillar Room. This started out well enough, with the evistros getting pushed into pillars and hacked down, but then the bigger demon arrives on the scene and things go downhill quickly (i replaced it with some lurker from the MM2).
They manage to escape with the Bell but the wizard is slain by the big demon and his body is pulled into a pillar. No one even saw him die.
So in the first venture to the Well they lost two characters and did not even bother trying to open the Sanctum and fight the Guardian.
They cut their losses and ran and let Sister Linora die. They couldn't save her (mission....Failed).
So, the next session they find out that Paldemar has a business deal with the gnoll priest and that the gnoll priest has a Silver Key that will lead straight to Paldemar's abode. Between that detail and wanting some payback for their slain friends, the heroes go back to the well for the second time.
But the Guardian has been released and it waiting in the front foyer, and this led to a very fun dragon fight along with over a dozen gnolls as well. Now that the Well of Demons has been rededicated to Yeenoghu many of the traps that were there for Baphomet and his petitioners have been disabled or altered. In fact, the heroes don't even need the Mask, Bell, Blade and Tome anymore.
But the central sphere of Annhilation is spinning around the central course, and the gnolls quickly learn that the PCs are back. Enemies swarm out of the final chamber and the big battle ends right outside the door to the Inner Sanctum. No one died this time, but it still featured several close calls. Ultimately the heroes defeat everyone, loot the treasure, and take the key to find Paldemar's Tower of Mystery.