Against the Darkmaster is Brilliant. It is like everything MERP was meant to be. If you are a MERP or RM fan, I can't recomend AtDM enough.

I do not have much to add except to confirm what has already been said by many about this adventure - it is one of the better modules that TSR ever published. It has a sword and sorcery kind of eldritch weirdness vibe to it, a place that should not exist. It is also one of the first and best "faction" modules. It is overwhelmingly tough if the PCs think they are going to hack their way through. They must choose sides and interact with the bizarre denizens if they are going to have a shot. It reminds me a little of I1, Dwellers of the forbidden city, perhaps my favorite module all time. My biggest criticism of B4, unlike I1, is that it takes considerable suspension of disbelief to buy into this strange locale. And like many early dungeon crawls, their seems to be no reason for some of the monsters to be here waiting for the PCs to kill them and take their stuff. This is also one of those modules where the players might ask "what the heck does this thing eat?" A modern GM who is used to story driven adventures might have some creative work to do before implementing this into a campaign. Still, with this bit of prep-work The Lost City is one of TSRs best modules.