Mud terrain, brown }. It can be walked on, but if you stand in one spot for long enough you will start sinking into the mud. You will be able to pull free if you are only a little stuck, but once you get more stuck you cannot, and if you cannot extricate yourself somehow you will eventually sink below the surface and suffocate.

This might best be implemented with a new “mud pit” trap, which gets created by standing on mud for too long and works similarly to a regular pit in that you are held in the same place by it for a while as you try to move out. Except a mud pit allows you to sink down and suffocate.