Return to and escape from Black Rock

Well the adventure went reasonably well. The after climbing down the ladder the half party finds itself in a room filled with periwinkle mist. They defeat the ogre which materializes out of it and add the paladin which materializes to the party (had to add the new player somehow). They move on to find in the next room to find a kobold wearing welder's goggles and a reptilian beast with glowing periwinkle eyes. The kobold turns to gas and flees sicking the beast on the party. The discovers much to its horror that the gaze of this beast turns people into periwinkle mist, but only the druid misses enough saving throws to be fully affected. They find several bottles of periwinkle mist carefully labeled things like "White Dragon" and "Frost Giant". They put the druid in an empty one. They progress on to the next room which has one door heavily guarded by a balista, barred from this side with a lighting bolt projector in front of it. The other door is just shut although closer inspection reveals that it is barred from the other side. The thief uses his tools to unbar the door. On opening the door they find a chamber with four iron cells on the left and a carriage equiped with four dragon headed nozzles on the right. In back is the kobold with welders glasses and another kobold with a whip. The party decides to talk, once the kobolds explain about the four nightmares behind the iron doors, the party decides to go a different way. They talk the kobold with welders glasses into disarming the traps in the previous room exit through the formally heavily trapped door into the great underway and reunite with the rest of the party sleeping in the kobold camp above. They retreat to a hidden spot and take a long rest (there is some stupidity with a captured kobold which I will not go into). They are horrified to find on their return to the underway a drow army storming through (guess they weren't the only ones interested in finding out how the traps for the door worked).

Final Battle Mat

Original keyed sketch

P.S. Although the players prudently exited before reaching the lava pool, I am bit disappointed not to see how 4th levels deal with lava.

What I did on my summer vacation

Well I appear to have taken the summer off. Staying step ahead of my players in my Tuesday 5e game seems to be taking most of my creative time. I am of course using this blog post to procrastinate finishing the lower levels of Black Rock, a strange twist of fate whereby my players always want to head into the areas I have not defined in my 20+ years of campaign notes. Most of the parties hearing about 400+ kobold armies and something called a "Fire Lich" would run the other way in a sandbox campaign, but not my players. After launching a frontal assault on a vanguard of thirty kobolds they successfully carved their way into the Black Rock itself, only to be met by the elite guard. The wizard was horrified to discover that his sleep spell only dropped one kobold. However they still carved their way through, only to be met by the next wave. A quick challenge to single combat with the CR 5 Kobold chieftain was accepted and although the principal fighter was defeated, her lizardman henchman was able to finish off the wounded chieftain. At that point the kobold shaman, his gaggle of kobold archers and elite kobolds asked them to leave. The cleric, the lizardman, and the fighter did. However the thief, wizard, and druid used a handy side passage and rope trick to lay low until the commotion died down, sneak into the depths of the Black Rock. I halted the session there as the deep depths of Black Rock are as yet undrawn. However, as the inner sanctum of a lich I have always envisioned them as  at least CR 6. So do I stay true to my vision keep it high level or do I tone it down so my half party of  4th level adventures can succeed?