## The *freedom of movement* spell only prevents difficult terrain from affecting our *movement*. The saving throw happens regardless but whether failing it does anything is unclear ##

The *freedom of movement* spell states:
> For the duration, the target's movement is unaffected by difficult terrain, and spells and other magical effects can neither reduce the target's speed nor cause the target to be paralyzed or restrained.

From that we can conclude what things the spell does:

1. Difficult terrain does not affect your *movement.* Note: it does *not* provide immunity to other effects of difficult terrain, such as changes to our speed, hit points, or anything else. 

2. Spells and magic effects cannot reduce your speed.

3. Spells and magic effects cannot make you paralyzed nor restrained. 

The Gibbering Mouther's Aberrant Ground feature is not magical, so the latter two points do not apply. What the feature does do is this: 
> The ground in a 10-foot radius around the mouther is doughlike difficult terrain. Each creature that starts its turn in that area must succeed on a DC 10 Strength saving throw or have its speed reduced to 0 until the start of its next turn.

1. Ground within a 10-foot radius becomes difficult terrain.

2. If a creature starts its turn within a 10-foot radius, it must make a saving throw; failing this saving throw makes your speed become 0.

The feature never says that being immune to the normal effect of difficult terrain makes us automatically succeed on (or not have to make) the saving throw. The feature never makes an exception to any sort of creature so *any* creature that starts its turn in the area will make the saving throw.

The terrain is still considered difficult terrain, and *freedom of movement* only prevents it from affecting our *movement*. **Simply making a saving throw is not affecting our movement so we would make the save no matter what**. If we were to fail the save, there are two possibilities:

1. This does not count as difficult terrain affecting our movement, instead it is the mouther itself; our speed, and thus movement, will reduce to zero.

2. This counts as the difficult terrain affecting our movement *through* affecting our speed; our speed will reduce to zero but our movement will remain the same. This is a strange, though not inherently impossible, situation.

This odd situation of having 0 speed but still being able to spend movement also shows up in this scenario as discussed in the question "[Is Freedom of Movement hampered from actually working to escape a grapple?][1]" 

There are also spells like *spike growth*, which create a damaging area of difficult terrain; *freedom of movement* will not somehow prevent us from taking damage; it only prevents the area from costing additional *movement*. This further supports that *freedom of movement* does not prevent *all* effects of difficult terrain, only ones which reduce our movement.

If *freedom of movement* prevented us from making the saving throw against Aberrant Ground I see no reason it wouldn't also prevent the damage from *spike growth*, which certainly seems outside the scope of the spell.

[1]: https://rpg.stackexchange.com/questions/46612