Cult of the Khryll (also known as Eye of the Titan) is the second part in the Cyclopean Deeps campaign and is loosely connected to the Rappan Athuk campaign. You don't need Rappan Athuk to play the campaign.
It describes the underground fortress of Ques Querax, as well as its catacombs.
From the publisher's website
The second adventure in the Cyclopean Deeps series by award-winning author Matt Finch. Eye of the Titan takes the adventurers to the strange, underground fortress of Ques Querax, where they may discover strange secrets if they are unlucky enough...