The level 5 spell Transmute Rock of Wizards and Druid can be used to Transmute Rock to Mud:
You choose an area of stone or mud that you can see that fits within a 40-foot cube and is within range, and choose one of the following effects.
Transmute Rock to Mud. Nonmagical rock of any sort in the area becomes an equal volume of thick, flowing mud that remains for the spell’s duration.
The ground in the spell’s area becomes muddy enough that creatures can sink into it. Each foot that a creature moves through the mud costs 4 feet of movement, and any creature on the ground when you cast the spell must make a Strength saving throw. A creature must also make the saving throw when it moves into the area for the first time on a turn or ends its turn there. On a failed save, a creature sinks into the mud and is restrained, though it can use an action to end the restrained condition on itself by pulling itself free of the mud.
If you cast the spell on a ceiling, the mud falls. Any creature under the mud when it falls must make a Dexterity saving throw. A creature takes 4d8 bludgeoning damage on a failed save, or half as much damage on a successful one.
If the ceiling is less than 40 feet high, can Transmute Rock be cast on both floor and ceiling at the same time, to get both effects from a single casting?
Best Answer
You can only transmute one contiguous area of rock
The targeting for this spell is a bit tricky. You don't select any 40-foot cube and then affect all rock or mud within this area. Instead, you select a specific area of rock or mud within range, and the spell transmutes only that area. Hence, you cannot affect both the rock ceiling and the rock floor in the same casting of the spell, unless there is a wall or pillar of rock connecting them into a contiguous area of rock. If nothing connects the floor and ceiling within your desired 40-foot cube, then they would be considered two different areas of rock, and you can only target one of them.