Gibbering Mouther

A gibbering mouther does not have allies. It has Intelligence 3, no language, and the social acuity of a sponge. The framing has to be a handler, and the handler has to be unhinged enough to keep one. The clean version is an aboleth's thrall, a mad wizard who breeds them, or a cult that drags one on a chain into ambush positions and lets the babbling do the work. The party is allied with the handler. The mouther is loose ordnance the handler points at the door.

Use it for one thing: clear a room of conscious enemies. Drop the mouther through the doorway, let Gibbering spread to the 20-foot radius, and watch a guard barracks dissolve into farce. Half the guards do nothing, a third wander into walls, and the rest swing wildly at each other. The party walks in afterward and cleans up. Blinding Spittle on the obvious spellcaster is the handler's request before the mouther goes loose, since the spellcaster is the one who could end the chaos with a single Silence.

The mouther will not distinguish between the party and the enemies, because the party is also within 20 feet of it the moment they enter the room. Tell the players this before they walk in. Give them rope to leash themselves to the doorframe, earplugs that grant Advantage on the save, or thirty seconds of Silence from the bard. The handler keeps the mouther on a long pole and a leather hood, both of which come off only after the party has cleared the threshold.

Once the room is clear, the handler hoods the mouther again and drags it back into whatever tank or sled it travels in. The party should feel like accessories to a war crime, which is correct.

Have the handler whistle a single rising note when releasing the hood. Players should flinch at that note for the rest of the campaign.

A gibbering mouther is a chaos engine masquerading as a CR 2 monster. The bite damage is irrelevant. The mouther's job is to make the party fight itself, dissolve the corpses, and turn a tidy corridor encounter into a slapstick disaster. Place it where the party cannot avoid coming within 20 feet of it, since that is the radius where Gibbering becomes a problem.

Gibbering is the whole encounter. Every creature that starts its turn within 20 feet rolls a DC 10 Wisdom save, which sounds easy until you realize the party will roll it five or six times across the fight and someone will fail every round. On a failure they roll d8: half the time they do nothing, a third of the time they wander in a random direction, and on a 7 or 8 they swing at a random adjacent creature, which means the fighter punches the cleric. Roll the d8 in front of the players. The randomness is the horror.

Open with Blinding Spittle on the back-line cluster: DC 10 Dex save, 7 radiant in a 10-foot sphere up to 30 feet away, Blinded until the end of the mouther's next turn. Blinded targets attack with Disadvantage and grant Advantage to the mouther's bite, which lands at +2 for 7 piercing and Prones any Medium or smaller creature that drops. The bite's grim clause is that any creature reduced to 0 HP by it is absorbed: gear remains, body does not. Aberrant Ground turns the 10-foot ring around the mouther into Difficult Terrain, so the party cannot easily reposition out of the gibbering radius.

Mouthers do not retreat. They have Int 3 and Cha 6, no language, and one urge, which is to eat what is in front of them and burble while doing it. The fight ends when the mouther dies, which it will, in three rounds or so once the wizard remembers Silence shuts off Gibbering entirely. Reward the player who figures that out by ending the fight on the spot.

Roll the d8 for Gibbering effects in the open and narrate the random target by name. Players forgive a swing they didn't ask for. They remember it forever.