Shadow Keep and Flowing Water
I made a post asking people why they thought the Shadow Keep was flooded, and upon talking I realized some things. Most of us assumed it was flooded out of anger of Marika's abandonment, and this does make sense, but fails to explain one key detail.
It appears that the Keep was built with water removal systems already in place, as if drainage of large volumes of flowing water served some purpose, and the flooding is simply a result of that process being halted, presumably upon Marika's abandonment.
Some examples are: - the fact that there is a lever based drainage system with specifically made holes to drain the water from the Church District. These were here before the district was flooded.
the octagonal waterfall system below the Keep, where the coffin that takes us to Unte is located. This drainage system is clearly meant to carry water downward.
the many smaller aqueducts all over the lower portions of the keep, such as on the path to Unte.
It is for these reasons that I suspect the water systems may have to do with the fact that flowing water is symbolic of death in Elden Ring, and death rituals are clearly a part of the culture in the Keep. Other examples are the Mariner boats burning a sort of "grace flame" akin to the lamplight we heard about in the base game.
The presence of a specific new subtype of Vulgar Militiamen for the Land of Shadow, the Shadow Militiamen, is also interesting. They often show up in places tied to death which are forbidden, and are abundant here, especially near the burning boats.
It has been theorized that the Keep preceeds Messmer, and if it is the vestige of an older inhabitant, that older inhabitant entirely may have incorporated water based death practices as many of those that preceeded the Erdtree did (Rauh being another good example).
There are some interesting stories being told through waterfalls and flowing water in the DLC. There is definitely something interesting going on with the geography of them. One example is how the waterfalls that pour from the cracked in half Rauh pour down into the Ellac river, which cascades down nearly the entire land of Shadow, culminating in The Fringe (this is the term for the southern coast according to the OST, so both Charo's Grave and the Cerulean Coast). With this being the place where death washes up, there could be something here.
And also this river which begins at the Keep and leads us to the Abyssal Woods. There could be a story there too.