In addition I bought a copy of her 1982 card game, Jasmine: The Battle for the Mid-Realm, which you can see here on BGG. On the bottom side of the clear box for the game I spied a Skull Mountain-esque picture I'd never seen before on one of the cards. See above.
Darlene has a page about Idshoii on her blog, which verifies that it's not just a castle, but a carving at the top of a mountain:
"Ildshoii Castle, carved from shiny black obsidian rock, rose high above a crown of volcanos. The dark castle was the domain of Melantha, the ruler of Medrylthorn ..."
Update: Here is Darlene's own intro page for the game: