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Using the Door Sliding Behavior, set the ANGLE you want. 0 is up, 90 is right, 180 is down, 270 is left ... if I remember correctly. So, if you want your wall to slide down into the ground, set the angle to 180.
Intel i9-10900K 5.10GHz, 64 GB RAM, Nvidia GTX 1080 ti 11GB, Windows 10 64-bit, dual monitor display