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  • Writer's pictureKenny Webb

Monster: Sudden Brain

A disembodied wrinkly brain rushes forward, arcs of psionic power crackling in its wake and a short nerve bundled trailing behind it. The folds of the brain’s frontal lobe are furrowed, forming furious eyes.
Art by John MarkS.


Title: Sudden Brain

Short description: A quirk of improbability whereby a fully-formed levitating brain appears from the foam of nothingness, complete with a rich personality, false memories, psychic power, and (oddly enough) a mustache-twirling desire to be evil just for the fun of it.

Flavor text: Wherever probability leylines entangle, there is a chance, however infinitesimal, that suddenly a fully formed brain will Become. Given infinite timelines, sudden brains are inevitable.

Tags: tiny aberration, lawful evil

Such brains are unstable and tend to vanish almost as quickly as they appear. However, in these few moments, a Sudden Brain can achieve much to bring about their inscrutable designs. Most dangerously, a Sudden Brain may possess another mind, allowing it to persist.

Traits: hovers and flies, telepathic

Actions: create a psychic barrier, use telekinesis, possess another mind

5e Statblock

Armor Class 10 (20 when using a psychic barrier)

Hit Points 80 (32d4)

Speed 0 ft., fly 30 ft. (hover)

  • STR: 1 (-5)

  • DEX: 18 (+4)

  • CON: 10 (+0)

  • INT: 22 (+6)

  • WIS: 17 (+3)

  • CHA: 20 (+5)

Saving Throws Con +6, Int +11, Wis +9, Cha +11

Damage Immunities psychic

Senses truesight 120 ft.

Languages Common plus up to two other languages

Challenge 18 (20,000 XP)


Designer notes: Kenny Webb


The Sudden Brain is directly inspired by a bizarre thought experiment in physics and cosmology called the Boltzmann brain. The short version (to the best of my casual understanding) is that in an eternal universe with random fluctuations, literally any structure will eventually pop into existence if you wait long enough.

Entire planets, Volkswagen Beetles, Cheesy Gordita Crunches, and indeed, fully-formed human brains complete with false memories of a life that was never lived could spontaneously emerge from the void.

I can’t help but be tantalized by the idea of a hilariously evil brain emerging instantaneously from the quantum foam, convinced that the memories imprinted in its neurons are absolutely true.

Personality and Memories (GM Tip)

Though all Sudden Brains share the same cackling villainy, each Brain’s freshly formed personality and memories will be unique and will change how they interact with the world and what precisely they want.

Choose the Brain’s personality and memories ahead of time so you can prepare the encounter. You can create these completely from scratch, or you can use the tables provided. Bear in mind that the Sudden Brain is fully convinced that it has lived a full life up until this point and that its memories correspond to real events.

Depending on how the PCs approach the challenge, the Sudden Brain may fight them to the death in pursuit of its goals, or if the PCs can somehow convince them that they wish to help it in achieving its goals, it can be negotiated with.

When a Sudden Brain forms, choose two random personality traits and one random false memory from the following tables:


  1. Courteous

  2. Intuitive

  3. Logical

  4. Petty

  5. Self-righteous

  6. Power-hungry

  7. Loves puns

  8. Inquisitive

  9. Arrogant

  10. Cheerful

  11. Principled

  12. Greedy

  13. Resentful

  14. Eloquent

  15. Rude

  16. Nihilistic

  17. Flamboyant

  18. Cautious

  19. Endlessly deceptive

  20. Paranoid


  1. The PCs stole the soul of the Brain’s beloved and stored it in one of their items.

  2. The Brain is the former ruler of the lost Toy Imperium.

  3. The PCs know the location of the Singularity Crucible and the Brain must discover it.

  4. The Brain is a planes-traversing chef, pursuing immortality so it may have infinite time to perfect its omelette technique.

  5. One of the PCs performed experimental surgery on the Brain, stealing its body and cursing it to its present form.

  6. The Brain is the hyper-intelligent pet parrot of a tropical sea witch and must be reunited with her so their reign of chaos may resume.

  7. The PCs and the Brain were once friends and members of an evil council of supervillains, but the PCs have had their memories wiped.

  8. The Brain is an artificial intelligence tasked with acquiring as many collectible trading cards as possible.

  9. The Brain is a historimancer and has discovered that the PCs are reincarnations of world-ending conquerors from a forgotten epoch. They must be stopped at all costs.

  10. The Brain believes that all of reality is an illusion, that nothing is real, and that everyone is just a brain in a jar being fed false information.

5e Statblock

Sudden Brain

Armor Class 10 (20 when using a psychic barrier)

Hit Points 80 (32d4)

Speed 0 ft., fly 30 ft. (hover)

  • STR: 1 (-5)

  • DEX: 18 (+4)

  • CON: 10 (+0)

  • INT: 22 (+6)

  • WIS: 17 (+3)

  • CHA: 20 (+5)

Saving Throws Con +6, Int +11, Wis +9, Cha +11

Damage Immunities psychic

Senses truesight 120 ft.

Languages Common plus up to two other languages

Challenge 18 (20,000 XP)

Innate Spellcasting (Psionics). The Sudden Brain’s innate spellcasting ability is Intelligence (spell save DC 20). It can innately cast the following spells, requiring no components:

At will: detect thoughts, dissonant whispers, levitate, telekinesis

1/day: invisibility (self only)

Quirk of Probability. At the beginning of combat, roll 2d20, rerolling duplicate results. Until the end of combat, those results count as critical hits for all combatants.

Unstable Fluctuation. At the beginning of its turn, if the Sudden Brain is not currently using Possess Mind, it takes 5 damage. When it is reduced to 0 hit points, it vanishes back into the nothingness from whence it came.


Warp Reality. Each of up to 3 targets within 10 ft. of the Sudden Brain must succeed on a DC 18 Wisdom saving throw or take 21 psychic damage. In addition, the Sudden Brain fundamentally changes the physical environment within a 10-ft. radius of it. This change cannot directly damage the targets, but can provide tactical advantage to the Brain. For example, the Brain can extend fences, remove bridges, turn doors into solid walls, or change stone tiles to mud.

Legendary Actions

The Sudden Brain can take 3 legendary actions, choosing from the following options. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Sudden Brain regains spent legendary actions at the start of its turn.

Flight. The Sudden Brain flies up to 30 ft.

Psychic Barrier. The Sudden Brain cocoons itself in a barrier of psionic energy. It regains 10 hit points and its AC becomes 20 until an attack against its AC hits.

Possess Mind. One living creature within 10 ft. of the Sudden Brain must succeed on a DC 18 Charisma saving throw. On a failure, the Sudden Brain disincorporates, inhabiting the body of the target. If the target is a monster, they are under the effect of dominate monster. If the target is an NPC, they are under the effect of dominate person as if it were successfully cast by the Brain. If the target is a PC, they may still take actions, but on the Sudden Brain’s turn it may take any action it could ordinarily take in addition to any action the PC could take. Each time the PC takes damage and at the end of each of their turns, they may make a DC 18 Charisma saving throw to end the effect, causing the Sudden Brain to reincorporate in its original form in an adjacent space. While possessing another mind, the Sudden Brain will not risk harm to its host body if it can avoid doing so.


A quick note on how we approach 5e: Occasionally you might see statistics and other choices we make in our publications that make you say, “Huh. I wonder why they did it that way?”You can check out this page dedicated to explaining how we approach 5e.

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