Spoiler free post.
By design, because I don’t want to spoil, and by necessity, as I do not have the books.

Rappan Athuk is one of those well-known names that make those in the know slowly nod and rub their beards, and mumble things like “don’t go down the well”…

For longer than a year now I am in a party that returns to a game of DnD 5e every week and tries to probe the dark mysteries of this dungeon of weirdness.

However!

However, of Rappan Athuk itself we have not seen a whole lot. We only made it down to a small portion of level 3, and parts of level 1 that feel very incomplete, even though we did not see many options to go elsewhere than we did. Most of our exploration went on in level 2, and even that, extensively mapped, is not complete by a long shot.

Rappan Athuk is not alone either, as in the area are several other dungeons, crypts, and fallen fortresses, of which we have snooped out three in the time since I joined the party.

Is it easy or hard?

Is Rappan Athuk easier in 5e than it is in older systems?

I don’t rightly know. Very possible. But that does not make it easy as such.

Of the party of seven that we were when I joined, only two are left standing, and one was dead and preserved for months of in-game time before she could be revived. Mighty barbarians have fallen, got eaten by wolves, and nosy witches died in flames, whereas two wizards have quit while ahead.

The number of dead is higher than in any other 5e game I know of, which I blame on the setting, not the GM.

Is it lawful or chaotic?

The general drift of the group is that the clerically and paladinally inclined characters thrive best, especially since most of the opposition is made up of stinking worshippers of foul chaos gods and demons. Being a follower of the Lords of Law is definitely a boon to keep a true North. The rewards in simple coin are fewer than in the satisfaction of smiting evil, so it is clearly slanted in an anti-chaos direction.

Hobbit Luck (re-roll 1s in the party) and a Rogue with unnaturally high Passive Perception are great assets as support characters in the rear.

Does it have a good plot?

It does not have a plot. You make the plot. It is just places, filled with potential, names, some backgrounds to inform NPC play. The world around it and the size and number of dungeon levels, as well as the overland distances, make it so that there can only be a theme — that of the struggle against the chaotic gods that would ride roughshod over their enemies, the unaligned, and their followers alike — but the story is very strongly dependent on personal choice, and the ability to survive in dire situations.

The great deed of the opposition — destroying a mighty army of Law — happened in the backstory of the setting, long ago. The enemies don’t have any overarching master plan and no overlord. They just do their thing in their various temples and crypts and sometimes venture out to wreak general havoc, but they have no direction. Which is good, as they are many, and if they had capable direction they would take down the “Lost Lands” rather swiftly.

It is also a good strong lot of places, so that you always have a great number of options where you want to go, and the choice to turn back and try another way first, and, if you live long enough, come back later, stronger.

Is it good?

It is, as you will have picked up already, to my liking.
According to the GM, the book comes in at hundreds upon hundreds of pages and about a quarter metric ton of paper and contains impossible amounts of maps and descriptions.

And they are all useful, as in, the game makes its players want to win against those bastards, even though it is quite unlikely that we or anyone will ever make it to the lowest levels, even of the smaller dungeons, let alone those of Rappan Athuk. It is not one megadungeon, but network of dungeons.

Wear & Tear

Our particular party has gone on long by now, and there is some strain among the characters, and the players too begin to get worn down. Slogging through the same tunnels again and again would break any party. Luckily, Rappan Athuk has them covered by offering other dungeons to shake things up, and surface locations and encounters to give the claustrophoics among the players options to shine outside of dungeon crawling.

When does it end? Nobody knows. The book offers an abundance of items to throw at the player characters, enough for reaching high levels during the course of clearing only parts of four of the dungeons.

Victory Conditions

What if the campaign ends before we have “won”?

That will very likely happen. It is a megadungeon without a plot, only with plot seeds, and without a big bad, only loads of medium-big bads who may or may not be aligned with each other.

So we cannot win. There are too many underground structures to clear in one party’s lifetime.
We can only…

…Fight on!