I have been everywhere through the Spider Forest and the Flayer Jungle, revealing all the map for it that I have found, and I haven’t found the Great Marsh or its corresponding teleporter.
I read the Great Marsh is sometimes spawned as an optional area, with the Spider Forest and the Flayer Jungle connecting directly to each other, but I haven’t found the Great Marsh even as an optional side-path someplace. I’ve spent a lot of time looking for it, too.
Is that kinda weird?
The great marsh is exactly between the two, they are sort of a triangle.
So you might wanna search the area between these two regions.
The connection between them is very fluent, so you might not notice it while in combat. Especially from the spider forest, i more often than not overlook the “map-change”-fade-in and then see the mapname changing in the upper right corner after a few hundred steps.
I usually have the exact opposite problem, because the great marsh is actually an optional area with little to no benefit.
What i mostly aim for is finding the flayer jungle directly.
The triangle-trick helps me find the entrance of the flayer jungle because it needs to be connected to the great marsh as well and is upstream when looking from the town (so i already know the side of the “road” and that it is “behind” the marsh).
In your case you just need to backtrack that logic.
To my knowledge the great marsh is always on the map. It is optional though, so you have no need to visit it unless you are a completionist on the waypoint matter.
You don’t want to find the great marsh…
It is there, but yes, sometimes finding the opening of the path leading to it can be frustratingly tricky to find, depending on how the map generates for you, in the same way that I’ve sometimes found it difficult to find the path to the Flayer Jungle.
Keep looking; it’s there.
Not finding the Great Marsh is actually a good thing. The only reason I ever go there is to collect the waypoint. From town I basically just stick to the far left or right of the path. If I hit the Flayer Jungle I’ll go back to town ant try again on the other side.