The Monsters Know What They’re Doing

The brain in a jar—literally, exactly what it is—poses several interesting problems, along with a few unanswered questions.

For one thing, is the “natural armor” that gives it an Armor Class of 11 supposed to be the glass of the jar? We have to assume so, because a naked brain with a −4 Dexterity modifier would have to have some ankylosaurus-grade armor plating to give it AC 11. However, according to the Object Armor Class table in chapter 8 of the Dungeon Master’s Guide, a glass jar ought to have AC 13. At the same time, 55 hp is absurdly durable for even a resilient object of Small size.

It looks like the brain in a jar was designed to have a defensive challenge rating of 1/4 (55 hp, reduced by one level for the under-13 Armor Class), which suggests an offensive challenge rating of 5 to 8. I’ve never been sure whether one is supposed to use values or steps for fractional CRs when averaging DCR and OCR to get CR. If values are all that matter, then reducing it to 18 hp (the maximum for a resilient Small object) and increasing its Armor Class to 13 should make no difference in its CR. However, if CR 1/4 is considered “CR 1 minus 2” rather than “CR 1 divided by 4,” then the brain’s damage output would need to be increased by 6 to 18 per round to compensate for its greater fragility.

The brain in a jar’s OCR seems to hinge on how many targets it’s supposed to be able to target with Mind Blast, which is on a 5–6 recharge. To deal 33 to 38 damage per round (OCR 5), using Mind Blast one round out of three and Chill Touch the rest of the time, the brain in a jar must target five creatures. To deal 39 to 44 damage (OCR 6), it must target six. Per the Targets in Area of Effect table, also in DMG chapter 8, six targets makes sense for a 60-foot cone, so let’s assume that’s where the brain’s OCR comes from.

To juice that up to OCR 8, necessary for a stepwise average, we need the brain to put out an additional 18 damage—on average, meaning an additional 54 from Mind Blast alone. However, that 54 damage is divided among six targets, so dialing up the damage roll from 3d8 + 4 to 5d8 + 4 is all it takes. (Why 18 damage and not 12? Because ratcheting up the damage per round like that puts the brain in a jar in a new save DC category. Since its save DC is only 14, we have to reduce the OCR by one step.)

Now, 5d8 + 4 is a lot of damage for a level 3 party: It’s 26 damage on average, which will probably knock out all your nonmartials (although a typical level 3 wizard will have a 60 percent chance of taking no damage at all!), and on a fluke roll it could deal up to 44, which would kill everybody. No fun. So let’s not do that. Instead, if you’re a believer in stepwise fractional CR calculation, simply treat a more fragile brain in a jar as CR 2.

Next unanswered question: Is the brain in a jar’s 10-foot flying speed, with hovering capability, just for the brain itself, or is it for the entire jar? Without any other information to go on, implicit or explicit, we have to conclude that it’s for the entire jar. (I mean, it’s there in the name: “brain in a jar.” We’re not talking about a brain that can decide to go jar-free.) In which case, I don’t think one would ever encounter a brain in the jar simply sitting on a table. At a minimum, it has detect thoughts active at all times, trains it on whichever creature in view seems the twitchiest, and launches itself upward the moment it senses belligerence in that creature’s surface thoughts.

Also, the brain in a jar speaks the languages it knew in life—with what? It has no mouth parts. Fifth edition Dungeons & Dragons books are usually fastidious about specifying when a creature communicates via telepathy (see, e.g., the bodytaker plants, necrichor, star spawn emissaries and Ulmist inquisitors in Ezmerelda’s Guide to Ravenloft), and they’re also fastidious about specifying when a creature understands a language but can’t speak it. The brain in a jar stat block specifies neither. Maybe it taps out messages on the side of the jar in Morse code, or vibrates it like the edge of a wine glass. I have no better idea, other than just giving the thing telepathy, which seems like it was intended and simply forgotten. [[Dopey me—as reader travdoc42 points out below, this question is answered in the flavor text, though not in the stat block.]]

Anyway, even once we consider those questions settled, we have to ask whether simply blasting away with a psychic subwoofer and squeezing enemies’ hearts with the chill touch cantrip, which neither deals cold damage nor has a range of touch, is a brain in a jar’s style—and also how we’re supposed to make use of its peculiar jumble of Innate Spellcasting (Psionic) abilities.

Normally, a creature with robust Constitution and dismal Dexterity would be inclined to tank it out in melee combat, but for a brain in a jar, such a conclusion is nonsensical on its face. Being close to its enemies doesn’t benefit it in any way; every one of its capabilities is either ranged or area-effect. The disparity between its extraordinary Intelligence and its unexceptional Wisdom is also strange: It implies a creature that’s excellent at assessing its enemies’ weaknesses yet indiscriminate when it comes to choosing targets. One way to interpret that, I guess, is that its combat behavior is impulsive and reactive rather than carefully considered or even intuitively sound—but the moment it picks a target, it knows exactly what to do to them.

As for its spell repertoire, it should be viewed through the lens of the Detect Sentience trait, which senses thinking beings within 300 feet. Combined with the brain’s 120-foot blindsight, this trait suggests that the brain is aware of nearly any approaching creature and continually updates its assessments of what to do with them as they enter the range of each of its psionic abilities.

With that in mind, let’s sort them and sum up what they do:

Range	Ability	Sight required?	Effect
300 ft	Detect Sentience	no	The brain in a jar knows you’re there and where you are. You’re a blip on its radar that doesn’t go away.
90 ft	sleep	no	Tranquilizes 9d8 hp worth of creatures in a 20-foot radius (40-foot diameter) sphere, with at least two-thirds confidence of affecting 38 hp or less.
90 ft	hold monster	yes	Hold person for nonhumanoids.
60 ft	hold person	yes	Paralyzes a humanoid, who must make a Wisdom save to resist. Requires concentration, and the target humanoid gets another save attempt at the end of every turn. Thanks to the brain’s unimpressive spell save DC, nobody with a Wisdom save modifier greater than −1 needs to be afraid of it.
60 ft	zone of truth	no	Each creature must make a Charisma save upon entering a 15-foot radius (30-foot diameter) sphere. If it fails the save, it cannot tell a lie. The brain knows who’s failed and who’s succeeded, and those who’ve failed know they’ve failed.
30 ft	detect thoughts	yes	Reads one creature’s surface thoughts. Requires concentration.
30 ft	mage hand	no	Lightweight telekinesis.
30 ft	charm person	yes	The whammy. A charmed creature views the brain in a jar as a friendly acquaintance, calling into question its taste in acquaintances.
30 ft	compulsion	yes, and target must be able to hear the brain, too	Moves the target around like a chess piece. The target can’t be sent into an obviously deadly hazard, but a cage isn’t an obviously deadly hazard. Requires concentration.
30 ft	Tasha’s hideous laughter	yes	Like hold monster but sillier.

With everything sorted out in this way, we can develop a heuristic.

Within 300 feet, the brain in a jar knows someone’s coming, but until it sees them and/or can talk to them, it doesn’t know what they want. Still, it’s on alert.

Within 120 feet—the standard range of telepathy, if you decide to give it telepathy—it can start to communicate with its visitors. If you don’t give it telepathy, or if you prefer to limit the radius (say, to 30 feet) or targeting (say, to a creature the brain can see), nothing happens at this distance.

Within 90 feet, the brain in a jar can cast sleep if it wants, but while this ability could dependably drop a posse of up to nine commoners, it doesn’t work as well on adventurers, and the brain can tell when its visitors are the special kind (ETA: if it has telepathy—if it doesn’t, all it senses are blips). It also knows that the magically induced slumber doesn’t work on elves and fey and that it lasts only a minute, so it had better be prepared to deal conclusively with anyone it knocks unconscious in this way. (ETA: A brain in a jar without telepathy can use sleep as a probe, just to see what happens. If it doesn’t drop every visitor in the group, that fact tells it something about their nature. In other respects, however, it’s far from the highest and best use of a once-per-day ability.) The brain can also cast hold monster, but it only gets one shot against one monster, and again, it lasts 1 minute, tops. Also, therefore, not something to use except in case of emergency.

Within 60 feet, the brain can cast zone of truth, and this is where things get interesting, because the brain loves to gather information and cares a great deal about whether that information is reliable. (What’s it going to do with that information? Who knows? Nothing else in either the stat block or the flavor text gives us a clue, except for one thing: a single, oblique reference to its creator. Maybe the creator is still around, and the brain shares intel with it.) At this range, the geography of the brain’s location matters a great deal, because it affects the route its visitors will take to get to it and how close they’ll be when they first see it. The brain therefore doesn’t cast zone of truth right away; rather, it waits until its visitors reach a point where, in order to have a conversation with it, they’ll have to enter the spell’s area of effect. Sixty feet is also the range of hold person, but again, the brain knows better than to start what it can’t finish.

Within 30 feet, the rest of the brain in a jar’s abilities all come into play. Since zone of truth doesn’t require concentration, the brain can cast detect thoughts as soon as it can see its visitors—or, better yet, knows from its psychic radar that they’re about to come into view. Charm person doesn’t require concentration either, so it can—if it wants—put the whammy on any visitor in whom it detects hostile thoughts. But it has to be careful about doing that, because charm person, just like hold person, requires a Wisdom saving throw, and the brain’s spell save DC is only 14. In addition, elves have advantage on the save. Elves are a problem in general for a brain in a jar. Tasha’s hideous laughter is exactly as useful as hold monster, against the same types of targets. That leaves compulsion, yet another ability requiring both concentration and a Wisdom save, but this one at least has potentially interesting applications—for instance, as mentioned above, getting that one low-Wisdom character to march themself into a cage, which the brain can then use mage hand on its next turn to slam shut.

The brain in a jar’s biggest problem, aside from elves, is that using any of its psionic abilities, even one of the reliable ones, consumes an action that it can’t then spend on hurting anyone. And there simply isn’t any math that justifies, for instance, spending one turn’s action on hold person or hold monster to paralyze a creature in the hope of gaining advantage on the next turn’s Chill Touch. (It’s even worse with hideous laughter: causing the target to fall prone imposes disadvantage on Chill Touch, which is a ranged attack.)

Which brings us around to the brain’s one heretofore unmentioned asset: its halfway decent Charisma. It may lack proficiency in any social skill, but it still prefers conversation over combat (even if it has no qualms against employing such manipulations as zone of truth and charm person, which others might legitimately construe as assaults). For as long as it can, the brain in a jar seeks to get what it wants by asking for it.

Only when talk fails, or it detects imminent aggression in the thoughts of one of its interlocutors, does it suddenly switch gears and let loose with a Mind Blast, first positioning itself to strike at least six opponents (or all of them, whichever is less) in the cone-shaped area of effect, which covers about a 53-degree arc (round that to 60 degrees if you’re eyeballing it or using a hex grid). Once it’s made the switch, the brain in a jar goes full aggro, reusing Mind Blast whenever it recharges and otherwise falling back on Chill Touch.

With two exceptions: First, the brain in a jar is good at math, and it can rapidly calculate whether it’s less likely to hit with Chill Touch than its target is to fail a Wisdom save. The formula is AC > S_Wis + 14, where AC is the target’s Armor Class and S_Wis is their Wisdom save modifier. If the statement is true, the brain in a jar becomes willing to try casting charm person or hold person even if the target’s Wisdom save mod is greater than or equal to zero. (If you’re not as good at math as a brain in a jar, do the calculation in advance of your session.) Second, it’s equally good at threat assessment. Fleeing is a hopeless prospect for a brain in a jar, which if it exerts itself fully can waft away at a poky 20 feet per turn, but if it’s seriously wounded, or if it sees that all its foes are seriously wounded, it uses sleep as a defense of last resort, finishing off its conscious enemies before dealing with the unconscious ones. If that gambit fails, the panic-stricken brain tries its best to parley its way out of the corner it’s in.

Next: vampiric mind flayers.