DA3: City of the Gods
DA3: City of the Gods is Dave Arneson's second and last hurrah for adapting his 1970s Blackmoor adventures into BXCMI, with David Ritchie's help. (Arneson will get back to this setting two decades later, without Ritchie.)
This content mainly expands the lands south of the Blackmoor area map, wherein is the Valley Of The Ancients in which this "city" lies... crashed. Also around it, we get the Plains in which the last Peshwah are penned up; and the Rand, the northeasternmost edge of Duke Taha Markovic's holdings since Thonia lost the Blackmoor area to Uther.
The "new monsters" include the geonid from the Desert Nomad series, the garl from CM6: Where Chaos Reigns, and the roper (12 HD, here) and hook horror from the 1970s. The DA line isn't making CM7's mistake; this is for an Expert-level party, and where the Companion Set cannot be avoided, the details of CM spells like Lore and Mass Invisibility are brought forward here.
As an adventure, it's the sequel to DA2: Temple of the Frog - and a natural one. Blackmoor is aware of high-tech aliens and would really like to learn more. The party is not disappointed - it is a starship from SPESS full of robots and laser guns, even a lightsaber.
So far, so S3: Expedition to the Barrier Peaks - and, indeed, the City had almost certainly inspired S3. So now, Arneson gets to explain (more or less) how he'd designed the original.
That design is vast and open-ended, and complicated. The party is to thwart "Saint Stephen" of the Frog Temple, also known as Rocklin; who is first defending himself from the local fremen (a silicate race of desert bandits) and secondly trying to take the City / starship from its rightful captain.
The stakes are high, as a result - although if Uther has the City on his side, adventuring in Blackmoor is unbalanced to the point we're all basically done here.