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?