Curse of the Ancients is old classic tomb-raiding affair, in which you should "acquire" some old relics from another underground complex with Lost City aesthetics - while fighting (or avoiding) lots of undead and solving puzzles. What is different is that you start in haunted city section (in which you should gather some additional information about your targets) from which you may proceed to the tomb complex.
In general, Curse of the Ancients seems like mix of Haunted Cathedral's sealed section and Lost City. The undead-populated city section (funny detail is that ghosts are sounding and behaving themselves like they are ordinary people - whining, drinking, guarding) is first part of mission and feels like big prologue while underground adventures constitute the main body of mission. Some enemies could be extremely deadly since they use bows with fire arrows. The puzzles (in my opinion) are more or less comprehensible (though in one particular area you can soft-lock yourself - so save often - and don't forget about crates) at least comparing to some other similar missions (like Sepulchre of the Sinistral). Overall, quite the adventure. Perhaps, biggest flaw of mission are the huge empty underused spaces - rooms, squares, corridors... Also it is sad that author didn't release planned sequel to mission and the mission itself is now almost forgotten. Overall, Curse of the Ancients doesn't hold the candle comparing to Valley of the Burned or aforementioned Sepulchre of the Sinistral, but still is quite memorable on its own right.
I can't believe I haven't played that mission before. I finished it today and Curse of the Ancients is indeed a forgotten gem. I agree with you about the underused spaces and for me this is the only flaw of this mission. Otherwise it's a spectacular achievement and we need to remember it was made in 2000. I really enjoyed solving the puzzles in the tombs, they were pretty much straightforward and I never felt stuck for longer than a few minutes. Exploring the haunted city was a sheer pleasure as well.