First level was too tedious, I wasn't having fun so I stopped playing.
9 levels. This may be the most difficult scenario you will ever play.
Here's a review from the only person I know that completed it.
I just finished Ra'thor (except for the hidden level, which I did not find the entry point for).
"I've been playing Marathon since late '95, and think I'm a pretty good player, but your scenario is hands down the most difficult I've ever played. In fact, I'd say it's pretty near impossible to beat on Total Carnage, especially the last 3 vacuum levels. I freely admit that I cheated constantly for oxygen and once or twice for ammo (especially on Igor Ra'thor), and occasionally used Forge. I do have a life, and I just didn't have the time (or desire) to play those levels over and over again to reach the level of perfection needed to complete them on the available oxygen or ammo (at least what I could find). I believe I never cheated for shields, but I may be mistaken. It took me a long time to get through the maps, so memory fades. You made free use of hidden switches that made things that much harder".
D. Supanich, North Hollywood, CA (March 2003)