ZBrush Heightmap Projection Technique for Terrain
I learned this technique from my coworker Lara!
A few notes about the technique: You're free to sculpt on the terrain before or after projecting the noise if you want to add variation!
Also, for the projected noise, I do it on a wall, as opposed to a wall + floor. There are a few ways to make the floor match up, but you probably don't want to do the same deformation on the floor since you don't want a super rough floor for the player to walk on.
You could either subdivide the floor, and then target weld or similar to match the floor's edge to the geo of the top or bottom of the wall, and then clean the geometry up, or just subdivide then boolean your floor with the top of the wall and clean it up after.
Also, be very careful about seams in your UVs! Those seams will be visible in the heightmap, so it's important to try and be intentional with where they are, or hide them entirely!