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Image promptmedium ethereal aberration, emaciated hairless platypus-rat body, translucent flesh revealing a glowing pulsing brain, wide flat feeding bill agape, half-phased into the ethereal plane with blurred ghostly edges, cold psychic shimmer distorting the air, hovering silently, fog-bound ruin between planes, washed-out spectral blue-grey palette, faint crimson gleam in exposed brain, uncanny half-real menace, dark fantasy, grimdark, painterly digital art, dramatic chiaroscuro lighting, muted desaturated palette with corrupted-crimson accents, weathered post-Cataclysm atmosphere, intricate high detail, cinematic composition

Thought Eater

Medium Aberration, Unaligned
CR 5
AC 15
HP 97 (13d8 + 39)
Speed 30 ft., Fly 30 ft. (hover)
STR
7
−2
save −2
DEX
18
+4
save +7
CON
16
+3
save +3
INT
15
+2
save +5
WIS
16
+3
save +6
CHA
6
−2
save −2
Skills Perception +6, Stealth +7
Resistances Acid, Cold, Fire, Lightning, Thunder; Bludgeoning, Piercing, Slashing (from nonmagical attacks)
Immunities Psychic; Charmed, Frightened, Grappled, Prone, Restrained
Senses Blindsight 120 ft.; Passive Perception 16
Languages —

Traits

Ethereal Sight. It can see into the Ethereal Plane within 60 feet.

Etherealness. As a Bonus Action, it can enter or leave the Ethereal Plane. While Ethereal, it can see the Material Plane but can't interact with or be affected by it.

Mind Scent. It automatically knows the location of every creature within 120 feet that is maintaining Concentration or has a prepared spell.

Actions

Multiattack. It makes one Devour Thought attack and one Wasting Bite attack.

Wasting Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 13 (2d8 + 4) Psychic damage.

Devour Thought. Intelligence Saving Throw: DC 14, one creature the Thought Eater can see within 60 feet. Failure: The target takes 11 (2d10) Psychic damage and suffers one of the following (the Thought Eater's choice):

  • If the target is maintaining Concentration → its Concentration is broken.
  • If the target is not maintaining Concentration → the target can't take a Bonus Action or Reaction on its next turn.

Success: Half damage, no other effect.

Consume Preparation (Recharge 6). Intelligence Saving Throw: DC 14, one creature within 30 feet. Failure: One of the target's prepared spells — chosen at random by the Thought Eater — becomes unusable for 1 hour. The target immediately knows which spell this is. Spell slots are unaffected.

If the target has no prepared spells (e.g., a Fighter, Barbarian, or Rogue), it instead takes 1 (1d4) Psychic damage at the start of each of its turns for 1 minute and can't make attacks with Advantage.

Reactions

Phase Shift (3/Day). When the Thought Eater is hit by an attack, it can shift into the Ethereal Plane, gaining Immunity to that attack.

How to Use

The Thought Eater was originally a creature that removed a caster from the game — "it eats your spell slots." That is unacceptable in 5e: the caster player sits for three hours doing nothing, while the martial player is never affected at all.

My redesign — it hits everyone:

TargetEffect
Caster (Concentrating)Concentration breaks — their spell fades. Annoying but no slot loss.
Caster (not Concentrating)Loses Bonus Action + Reaction.
MartialSame: loses Bonus Action + Reaction. For a Rogue that means Cunning Action, for a Fighter, Second Wind, for anyone, an Opportunity Attack. This is a serious loss.
Consume PreparationCaster: a spell is locked for 1 hour (not a slot!). Martial: damage + loss of Advantage.

Result: now it's symmetric. Nobody says "this creature doesn't touch me," and nobody says "I'm completely out of the game."

Tactics: the Thought Eater hits and runs. It uses Phase Shift + Etherealness to strike and retreat. Cornering it is the party's job. Without a silver/magic weapon, damage is halved (Resistance).

Balance Note

I don't have Consume Preparation eat a slot — deliberately. Eating a slot steals the player's daily resource, and that never comes back. Instead I lock a single spell for 1 hour: the caster is still in the game, still has their slots, just their favorite spell is unavailable right now. This is a tactical problem, not a resource penalty.