How does the Gibbering Mouther's random-attack effect work against characters with multiple attacks?

Question

I have a party of 13th level characters, one of which happens to be a fighter who loves to swing her dual hammers at anything within reach. Recently I've been experimenting with some custom monsters and they are going to encounter a Gibbering Mouther/Minotaur hybrid, but before they get there I want to clear up a question I had.

Gibbering Mouthers have a trait called Gibbering, which reads as follows:

The mouther babbles incoherently while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the mouther and can hear the gibbering must succeed on a DC 15 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does its turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a randomly determined direction. On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.

The part I was particularly interested in was the last part, where a creature who rolls a 7 or 8 makes a melee attack against a random creature within range.

Given that the fighter has 3-4 attacks normally, not including her Action Surge which can effectively double that, how many attacks should she be randomly making? Is she effectively using the full extent of her Attack action to swing wildly, or would she only make the 1 attack and call it quits?

Answer 1

The Fighter makes a single attack and does not take the Attack action.

Compare all of the following wordings:

The Gibbering Mouther's Gibbering trait:

[...] On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.

The confusion spell:

The creature uses its action to make a melee attack against a randomly determined creature within its reach.

The crown of madness spell (PHB, p. 229):

The charmed target must use its action before moving on each of its turns to make a melee attack against a creature other than itself that you mentally choose.

Note that for these effects, a creature makes a melee attack, not "as many melee attacks as it could make if it took the Attack action". Thus it only makes a single attack, no matter what.

This is further supported in answers to the following question:

• "Is a player character required to use its Extra Attack when Charmed using Crown of Madness?"

However, the feature does not specify what kind of melee attack they must make, so if targeting an ally, they could simply make an unarmed strike and go full-force if targeting an enemy. This is supported in answers to the following two question:

• "What decisions can a player make when performing a compelled attack?"
• "When affected by the Crown of Madness spell, can I choose what type of melee attack to make?"

The Fighter actually does not take any action

Note that the Gibbering Mouther's effect has a slightly different wording; it does not say the affected creature uses their action to make the attack, thus the attack does not actually use their action. That said, the d8 result determines the entirety of what an affected creature does on its turn, and so making an attack is the only thing the affected creature does. This difference in wording may be a mistake unnoticed by the developers, though it has not been corrected by errata yet. That said, there are incredibly few times this wording difference has any meaningful effect on gameplay.

Answer 2

You make an attack, not an Attack action

An attack is not an Attack action; they are two different things.

Taking the Attack action allows you to make an attack:

With this action, you make one melee or ranged attack. See the "Making an Attack" section for the rules that govern attacks.

Extra Attack allows you to attack additional times when you take the attack action:

you can attack twice, instead of once, whenever you take the Attack action on your turn.

Each attack plays out according to the rules for making an attack:

1. Choose a target.

2. Determine modifiers.

3. Resolve the attack.

In the case of Gibbering the ability clearly says to make an attack, not an Attack action:

On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.