Gibbering Mouthers have two area-of-effect abilities:

Aberrant Ground. 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.

Gibbering. 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 10 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 during 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.

Are other gibbering mouthers immune to either of these effects if they are within range? For that matter, is a Gibbering Mouther automatically immune to it's own effects?

  • \$\begingroup\$ Having recently been attacked by a group of these, this is an amazing question and could have made the fight a lot more memorable - if potentially easier. On the easier question - would the fight xp budget have to account for them affecting allies? Is that already accounted for? \$\endgroup\$ – SeriousBri May 31 '19 at 8:52

According to RAW they are not immune to each other's effects. The abilities specifically say "each creature" and not "enemies", "allies", etc. - so any creature within range, no matter the species, will be affected.

In the Monster Manual (p. 157), under "Mouths of Madness", it states (emphasis mine):

When a gibbering mouther senses prey, its mouths begin to murmur and chatter, each with a different voice

This seems to indicate that the Mouther is able to hear, and so it would be affected. If it could not hear, and since it only has darkvision and no other sense such as blindsense or tremorsense, it should say "sees" instead of "senses" if it could not hear.

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    \$\begingroup\$ Can you confirm they can hear? \$\endgroup\$ – NautArch May 30 '19 at 22:53
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    \$\begingroup\$ Probably worth pointing out that according to the lore, Gibbering Mouthers consider any other creature food, and are thus unlikely to cooperate with other Gibbering Mouthers. If two Gibbering Mouthers were to meet, one would eventually consume the other and you would then have one larger one. \$\endgroup\$ – cpcodes May 30 '19 at 23:14
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    \$\begingroup\$ The question also asks whether the mouther's abilities affect itself, which is a much more interesting and important issue. \$\endgroup\$ – Miniman May 30 '19 at 23:39
  • \$\begingroup\$ You do mention if it can't hear, but if it can't then the gibbering won't affect it. \$\endgroup\$ – NautArch May 31 '19 at 10:58
  • \$\begingroup\$ @miniman It would say so if it self linflicted, as the general form of what monsters do in the MM is inflict on others ... plain English reading and all that. \$\endgroup\$ – KorvinStarmast May 31 '19 at 16:09

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