I really like this idea. Having it attached to one of the Thrundarr quests is thematically appropriate. You could even just put an extra up stairs in the Ogre Cave or something.
I'm trying to think of a nice way that it would be also accessible to CKs. Maybe speaking to Kherab would also reveal it? (uSe ThIs SeCrEt TuNnEl tO sNeAk InTo ThE hOmE oF mY mOrTaL eNeMy). There would be an interesting high-risk option of skipping the entire early CoC if you can get to Kherab early...
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.