My first comment has to be..... what a monster of a book! Over five pounds (!) of one of the biggest dungeon crawls I have =ever= seen!
Divided into three "books" - "The Desolation", "The Temple City of Orcus", and "The Hidden Citadel" - and a slightly smaller "bonus" chapter called "Sleeping Dogs" - this has to be one of the very largest books for Role Playing Games ever. There are a few others, such as "Rappan Athuk", "The Blight", and "The Northlands Saga" that come close. This version of "Slumbering Tsar" was put together out of smaller modular parts - if I remember right, it was 15 pieces!
At best, I call these Magnum Opuses "Mega-Dungeons". Meant, in most cases, to take a role playing party of adventurers from 1st level to 20th and possibly beyond! TSR has some of these now too, such as most of their 5th edition hardcovers like: "Curse of Strahd", "Hoard of the Dragon Queen", "The Rise of Tiamat" and others. But Necromancer Games and Frog God Games beat them to the punch!
I've got to admit that I was blown away by just the sheer size of this tome - 960 pages of nothing but dungeon crawl crunch and great illustrations. In the first section, "The Desolation", you are taken from your first base camp to a crossroads - one of which leads to "The Slumbering Tsar" and the other one leads to "Rappan Athuk"!
Upon reaching "The Desolation", the party sees the outskirts of The Slumbering Tsar - an area of blasted ruins that surrounds the supposedly sleeping city. Just using the 'feel' from this place should give players a warning that they are approaching a very evil place indeed. After about nine days of travel with plenty of adventures through the blighted area, they reach the entrance to the 2nd section - "The Temple City of Orcus"!
Although about a thousand years have passed since the Temple City was supposedly destroyed, DM's should =really= play up the area around the entrance as being extremely dangerous. If the players decide to go further, after many encounters with evil monsters both human (mostly Cultists and the like), and non-human (like the many creatures and monsters that inhabit the area since just after the ruination of the Temple City), the party would be wise to make a second camp near the entrance to the underground areas. If they don't keep a =very= good watch, the deserve what they get.
Entering the Hidden Citadel should really tax any character to their limit! Once inside the Temple City, characters are taken on a roller coaster ride through various ruined temples for worshipping evil deities, and the old city that surrounded them. If the party isn't dead by then, there is always the 3rd "book" - "The Hidden Citadel. "
The final object of this, believe it or not, is to eventually challenge Orcus himself! If, by some miracle, the party manages to "kill" Orcus, GM's should remember that the only way to really destroy Orcus forever is to challenge him on his home plane. If the party succeeds in dispelling Orcus, the first thing a good GM should remember is that Orcus, like any other Demon (or Daemon, or Devil) is that he can remanifest himself on this plane after a year and a day - and he will be hopping mad at the characters that did it!
Folks, I cannot say enough good about this huge saga. I was really impressed by the way that this book allows a GM to raise a party from essentially 1st level to 20th level. It is very obvious to me that this is not your usual 'cakewalk'. It is deadly in the extreme - filled with high level monsters and creatures, and bunches of traps. If you can handle the huge price tag ($150.00!) it is absolutely worth the money. One of my very highest recommendations.
Now that you've read all of this about one super mega dungeon, let me remind you that "Slumbering Tsar" is on the same world as "Rappan Athuk" - another mega-dungeon!
My compliments to Frog God and Necromancer for this masterpiece!