Hit Dice: 12d8+72 (126)
Speed: 10ft, swim 20ft
Armor Class: 20 (+1 Dex, -1 Size, +10 Natural.)
Base Attack/Grapple: +9/+18
Attack: Bite +13 mêlee (1d2+5.). Spittle +3 ranged touch (1d4 acid + Blindness.)
Full Attack: 6 bites +13 mêlee (1d2+5.).
Special Attacks: Gibbering, Spittle, Improved Grab, Blood Drain, Engulf, Ground Manipulation.
Special Qualities: Amorphous, DR 5/Bludgeoning, Darkvision 60ft.
Saves: Fort +10, Ref +5, Will +9
Abilities: Str 20, Dex 12, Con 22, Int 2, Wis 12, Cha 18
Skills: Intimidate +13, Spot +8, Swim +8
Feats: Ability Focus: Spittle, Ability Focus: Gibbering, Martial Study: Shadow Jaunt, Frightful Presence, Reverbation.
Challenge Rating: 8
Alignment: True Neutral
Gibbering (Su): Free action, each round. Everyone within 60ft spread, DC 24 Will Save or confused for 1d2 rounds. (Sonic, Mind-affecting, compulsion.) Creature that saves is immune for 24h.
Spittle (Ex): Free action, each round. Ranged touch to someone within 30ft. 1D4 damage, and target must succeed on DC 24 Fort save or be blinded for 1d4 rounds.
Improved Grab (Ex): Start grapple as free action after hitting with a bite attack.
Blood Drain (Ex): On successful grapple, mouth attaches to opponent. Automatically deals bite damage and drains blood, dealing 1d2+5 damage and 1 constitution damage each round. A mouth can be ripped off (Dealing 1 point of damage.) or severed with a successful sunder attempt. (2 hp.). A severed mouth continues to bite and drain blood for 1d4 rounds after such attack.
Engulf (Ex): Can try to engulf a medium or smaller opponent grabbed by three or more mouths. Target must succeed on a DC 23 Reflex save or be engulfed. Next round, twelve bite attacks. Engulfed creature cannot attack the mouther from within. The previous mouths are now free to attack others.
Ground Manipulation (Su): At will, standard action. Cause stone and earth in adjacent squares to become quicksand. Anyone other than the mouther in that area must take a move-equivalent action to avoid becoming mired. (Treat as being pinned.)
Amorphous (Ex): Is not subject to critical hits/sneak attacks. Cannot be flanked.
Frightful Presence (Ex): Free action, each round. Everyone within 10ft must take DC 20 Will save or become shaken, taking -2 penalty to attack rolls, saves, and skill checks for 1d6+4 rounds. On successful save, immunity for 24h.
That is an advanced Gibbering Mouther I sometime whipped up to annoy my players. It is pretty dangerous. When I say that, I mean that they're probably fucked if you send a few. Then again, your party has ridiculously many players, so just send something that can actually deal damage, too. Like...
Bearded Devil Warblade (Enraged)
Medium Outsider (Baatezu, Evil, Extraplanar, Lawful.)
Hit Dice: 8d8+1d12+63 (105 hp)
Armor Class: 18 (+5 Dex, +7 Natural, -4 Untyped.)
Base Attack/Grapple: +8
Attack: MW Glaive +18 melee (1d10+1d6+10 and Infernal Wound.) or claw +16 melee (1d6+1d6+7.)
Full Attack: MW Glaive +18/+13 melee (1d10+1d6+10 and Infernal Wound.) or 2 claws +16 melee (1d6+1d6+7.)
Space/Reach: 5ft/5ft (10ft with Glaive.)
Special Attacks: Infernal Wound, Beard, Battle Frenzy
Special Qualities: DR 5/silver or good, Darkvision 60ft, Immunity to Fire and Poison, Resist Cold and Acid 10, See in Darkness, SR 20, Telepathy 100ft, Battle Clarity, Weapon Aptitude, Combat Reflexes
Saves: Fort +14, Ref +11, Will +9
Abilities: Str 24, Dex 20, Con 24, Int 6, Wis 12, Cha 8
Feats: Improved Initiative, Power Attack, Weapon Focus (glaive), Rapid Assault, Sudden Recovery
Challenge Rating: 7
Alignment: Lawful Evil
SLA's: At will-greater teleport (Self and 50 pounds only.) CL 12
Infernal Wound (Su): Any subject wounded takes 2 additional damage each round. Does not heal naturally, resists healing spells. DC 18 heal check or cure spell stops, but cure spells and heal spells on subject must succeed on a DC 18 caster level check, or the spell has no effect.
Beard (Ex): Hits with both claws, hit with beard. 1D8+1d6+7 damage, and a DC 19 Fort save or get infected by Devil Chills. (Inc Period 1d4 days, damage 1d4 str.) Three consecutive fort saves stop it.
Battle Frenzy (Ex): 2/day, Barbarian Rage, lasts 8 rounds. Not fatiqued after. +4 Str, +4 Con, +2 Morale on Will, -2 AC. Stats above given for enraged creature.
White Raven Tactics
Leading the Attack
Rapid Assault: +1d6 damage on the first round of combat.
Sudden Recovery: Recover expended maneuver as a swift action 1/day.
Yeah, two of the mouthers and four of these and you probably got yourself covered. Let them have fun with White Raven Tactics and the mouthers so they can mire everything, and then let them recover that ability and use it again. Then once your party has gone crazy for the gibbering, run in and hit them.
These both are, as far as I know, completely RAW creatures for their CR. The devil has the extra feats using the "remove summon, give feats" method from Fiendish Codex II.
Should we have a "Fuck your players over"-handbook for DMs? I have a bunch of encounters like these on my computer. I tend to make new ones when I'm bored, too.