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These brain-dog-looking things first appeared in the original AD&D Monster Manual. This is their 4e debut.
Intellect devourers are psychic predators that evolved in the Far Realm and spread to the deep places of the world. They roam the underground and the Underdark, hunting other creatures using their psionic powers and draining their mental energy until only an empty husk remains.
These monsters reproduce by forming larvae from the brain tissue of their slain victims. These larvae are known as ustilagors, and they hunt in packs until their first metamorphosis, which turns them into the adults known as Intellect Predators. Predators who manage to feed well and survive long enough undergo a second metamorphosis into a larger and more powerful form, known as an Intellect Glutton or Brain Collector.
Each form is smarter than the last, though even ustilagors are dimly sapient. Intellect devourers can communicate via telepathy, but they don’t appear to have much in the way of a culture. They see other sapients as food, not as conversation partners.
All intellect devourers are Aberrant Magical Beasts with the Blind keyword. Blindness is an advantage in their case - they’re immune to gaze attacks and to the blinded condition. They can perceive the world and fight PCs just fine with their Blindsight 10.
Their other signature trait is Mind’s Resilience. If they take damage while under an effect a save can end, they can roll a save immediately.
Ustilagors are Small devourer larvae. They’re individually weaker than an adult, but they hunt in packs and are smart enough to coordinate. And as Level 7 Lurkers with 59 HP, they’re still pretty deadly. They have a ground speed of 6.
Ustilagors will first try to secure combat advantage against their chosen victim. Attacking from ambush will do this, but if that’s not possible they can do this in two other ways. First they try Cloud Thoughts, a minor action attack that does no damage but makes the ustilagor invisible to the victim (save ends). If that fails they can use a Thought Lash, a ranged attack that does psychic damage and forces the target to grant combat advantage for a turn.
Once it has CA, the Ustilagor will attack with Clinging Menace, which allows them to jump 4 squares without provoking attacks of opportunity and make a melee attack. This attack does very little damage, but it allows the larva to grab the target. While the grab lasts (Athletics DC 19 or Acrobatics DC 17 to escape), the victim is dazed and takes 5 ongoing damage.
Ustilagors can also fight with their Claws, which are decent basic attacks. Those will likely be used to fend off the grabbed victim’s allies if they try to interrupt the meal.
This adult specimen is Medium. It has learned the most famous, and scariest, intellect devourer technique: psychic possession. As a Level 14 Controller with 140 HP, it’s quite dangerous. It has a ground speed of 8.
Intellect Predators will try to open the fight with a Thought Lance (recharge 5+), a ranged attack that does no damage but stuns for a turn on a hit. If they fail to land this, they’ll use their robust Claws until they can try again.
If the lance hits, they can use Body Thief on the stunned target. The attack does psychic damage and allows the predator to merge with the target, dominating them. Since the monster is merged, it can’t use its claw attack and occupies the same space as the target. Normally dominated targets are also considered dazed, so they can only take a single action on their turn. However, the predator can grant them an extra move or minor action with the Puppet Master ability, which costs the monster only a minor action. So for all intents and purposes the dominated target moves normally. They have a -2 to the save against this possesion, and when it finally ends the predator appears in an adjacent space.
If the predator is surrounded it can use Mind Shock to attack every enemy in a close burst 5. On a hit this deals a bit of psychic damage and slides the enemy 1 square, which can open a way for the monster to run away.
Intellect Gluttons are also sometimes called Brain Collectors. They used to be a different monster in earlier editions, but now they are a particularly large and old specimen of intellect devourer. Their advanced age only makes them stronger and more dangerous, since they had that much longer to absorb psychic energy.
Their bodies are protected by a thick translucent carapace with markings that resemble human faces, which leads some to speculate that they extract people’s brains and physically absorb them. This isn’t true, but they’re still terrifying. A glutton can use its enhanced abilities to puppet victims remotely, and to control the bodies of the newly dead.
Gluttons are Level 21 Controllers with 196 HP. They have the same Speed 8 of their younger relatives, and gain Spider Climb 6. They’re surrounded by a wide Thought Static aura (5) that inflicts a -2 penalty to Will for all enemies inside.
Their Claws do average physical damage, but it will definitely prefer to stay a bit further away and target people inside its aura with psychic powers. Thought Feast is a non-damaging attack that dominates a target (save ends). The dominated target takes 10 ongoing psychic damage as the glutton feasts on its thoughts, and the monster itself is insubstantial while this effect lasts. Mockery of Life allows it to control a dead creature. It hits automatically (the target is dead!), and makes the creature regain 20 HP and become dominated. This lasts until the end of the encounter or until the creature drops to 0 HP again. Both abilities recharge whenever the monster is not dominating a target with either of them.
The glutton also has a simpler ability named Mind Rend, a ranged attack that deals psychic damage and slides the target 1 square. Its main advantage is that the glutton can use it once per round as a minor action if it has a creature dominated.
The above powers mean that gluttons will try to have both a living and a dead creature dominated whenever they can, and when that happens they’ll use Mind Rend twice per turn to support the actions of their thralls. Having a convenient corpse lying around the battlefield as the fight starts is a way to speed this process up. Otherwise the most likely dead target is going to be one of the other monsters in the encounter when it dies.
Puppet Master makes the Intellect Devourer into one of the scarier dominator monsters I’ve seen, because the victim gets full turns and the monster is not targetable while this lasts. Make sure you don’t use more than one or two of them in a single fight, or things are going to get very annoying very fast. Save-granting powers and items are a vital tool when fighting them.
Gluttons are actually a bit less annoying to handle since you can still attack them while they dominate someone. Mockery of Life is a wonderful power to use on dead artillery enemies after the PCs have moved away from the corpse.
Intellect Devourers aren’t exactly geniuses despite all the brain theming. They do get smarter as they get older, though, and I think they’re always considered sapient. Considering their level and preferred environments, I think it’s plausible to meet a pack of Ustilagors in the same caves where you’d find troglodytes. Adult devourers might be allied with a balhannoth or with mind flayers, since they eat thoughts, not brains. And I could see a glutton making deals with a colony of swordwings: “I get their brains, you take their stuff”.