True story: A few weeks ago, I accidentally left a carton of eggs in the trunk of my car during a heat wave, and I did not discover my mistake for a week. It took forever to get the smell out, and it was quite embarrassing. But now I don’t feel so bad about it because thanks to Phandelver and Below: The Shattered Obelisk, I learned that even illithids are guilty of it!

When an illithid colony leaves a bunch of eggs near the Far Realm (which is a lot like a hot car trunk, if you think about it), the consequences are far more dire than a rancid trunk smell. The eggs in question could possibly mutate into encephalons—the illithid’s most recently discovered “whoopsie-daisy.”

And you’re going to need a lot more than Lysol and Febreeze to clean up after these hungry, spawn-spewing slimeballs.

• Origins of Encephalon Clusters and Gemmules
• Key Traits of These Aberrations
• How to Include Them in Your Campaign

Origins of Encephalon Clusters and Gemmules

Artist: BRIAN VALENZUELA

Despite their weird nature, even aberrations aren’t immune to the inexplicably strange effects of the Far Realm. If an illithid colony puts down roots near, or within, the Far Realm, its eggs can mutate and fuse together into an encephalon cluster—a large, brain-shaped, moving mass that consumes anything it can envelop within its slimy, egg sack-covered body.

Unlike the neothelid, a single creature that will always be the end result of a neglected brine pool, encephalon clusters are a fairly unpredictable mutation… and they can multiply like moist, carnivorous tribbles.

When an egg attached to the cluster matures, an encephalon gemmule pops out. These tiny, psionic, pyramid-shaped chunks of flesh may be strangely adorable, but they do not bode well for an illithid colony. A cluster can spew forth several of these gemmules per day, and they mature into full-blown clusters within a month or so of being hatched. If left unchecked, encephalons can easily overwhelm an entire illithid colony like a virus before spreading to fleshier pastures.

Key Traits of These Aberrations

You may be wondering how a boulder made of melon balls and its offspring of cute little brain slugs can so easily dismantle an illithid colony, and that’s fair. I mean, if you look at something like a neothelid—which is a giant, angry worm hell-bent on consuming brains—you easily understand why mind flayers would be terrified of them. So what exactly makes these squishy little mutants so good at being gross?

First, let’s take a look at the key traits of the cluster.

Artist: HINCHEL OR

Encephalon Cluster Traits

Encephalon clusters have a relatively low Armor Class and movement speed, and they also have extremely low Charisma and Intelligence. But they more than make up for these shortcomings by being extremely durable and perceptive. A cluster has blindsight of 60 feet, giving it the ability to sense a farting mouse from 20 yards away. It also has resistance to psychic damage, magical resistance, and it has three legendary resistances per day. These things are literally tailor-made to shrug off most of a mind flayer’s offensive repertoire.

Additionally, clusters have two Slam attacks that can do a healthy sum of bludgeoning and psychic damage before forcing its victims to pass a DC 18 Strength saving throw or be knocked prone. And if a cluster manages to reduce a creature to 0 hit points, the creature automatically dies and is enveloped by the cluster as a yum-yum treat.

“Okay, so I’ll just stick to the sidelines and attack at long range, right?” Yeah, no, that won’t work either.

After being attacked, a cluster can use its reaction to move its entire movement speed toward its attacker without provoking opportunity attacks, and it gets a free Slam attack if it gets within 5 feet of the poor soul who attacked it. Though its blindsight only extends to 60 feet, so if you want to attack at long range, it needs to be…very far away.

Lastly, every long or short rest, a cluster can spew forth one to four eggs, which instantly transform into gemmules. Oh, that’s not so bad, right? Just stomp on those tiny little Far Realm Funko Pops and be done with it, right? Nope. Gemmules are just as insidiously hard to kill as clusters.

Encephalon Gemmule Traits

An encephalon gemmule can move twice as fast as a cluster, has blindsight of 30 feet, and magic resistance. It has a psychic slam attack, but its grossest ability is called Leech. If a gemmule attaches to a host, the host automatically takes 3d4 piercing damage at the start of its turn unless it uses an action to succeed on a DC 15 Strength check to pull it off. Can you imagine if two or three of these things were attached to you at once? Yeah. Nasty.

But in my humble opinion, the most terrifying aspect of the gemmules simply comes down to their size and their amount of hit points. These are pixie-sized aberrations with 54 hit points. Yes. Fifty. Four. And remember, a cluster can create one to four of them per short rest. If just one of these durable little horror nuggets escapes down a vent, it will grow into a cluster in a month or so.

Yeah, if I was a tentacle-faced brain nerd, I would be terrified of these things too.

How to Include Them in Your Campaign

Artist: LILY ABDULLINA

If your campaign heavily features illithids as antagonists, the introduction of encephalons can create an interesting dilemma for your players. Is the enemy of your enemy still your friend if it's basically a mindless, super-sized virus?

An Unstable Rift

Your story could begin with your players discovering an illithid colony rotting underground. At first, they believe it could’ve been githyanki or maybe even drow that razed this colony so effectively. But upon further investigation, they discover an unstable rift to the Far Realm and a vast infection of encephalon clusters surrounding it…

An Infection at a Colony

Perhaps your players are approached by a mysterious drow who is looking for capable (and expendable) warriors to deal with an underground illithid colony that’s recently collapsed due to an encephalon infection. If the colony is not completely razed within a certain number of days, the encephalon gemmules will mature into a hoard of clusters and spread to a nearby drow city. The cunning drow correctly points out that if their city falls, its only a matter of time before the infection reaches the surface…

Weapon of Mass Reproduction

Maybe your players have been tasked with razing a healthy, well-developed, illithid colony. But in the process, a cut-throat githyanki ally of theirs gives them a gift: a gemmule trapped in a bottle. Theoretically, your players could deal a devastating blow to the colony if they infiltrate it and plant the gemmule somewhere discreet where it can grow and mature into a cluster. If the githyanki in the Forgotten Realms are willing to make deals with Tiamat to kill mind flayers, they most certainly aren’t above a little biological warfare either…

Your players may not even need to release the encephalon gemmule. They could just threaten the colony as a whole with it.

A Cry for Help

There is also an unexpected turn you could employ when using encephalons in your game. Your players could be approached by a desperate, lone mind flayer who has resorted to seeking help from less-evolved entities to save its colony from infection. Because, for whatever reason, encephalons are evolutionarily predisposed to elude the primary weapon of illithids: psychic damage. If threatened with an infection such as this, seeking non-illithid help would be a perfectly reasonable strategy for an elder brain, or even just a single rogue mind flayer, to employ. Perhaps your players could even come to an agreement with the elder brain, “If we root out the encephalon infection, you must leave this region, release your uninfected prisoners, etc.”

Mind Flayers Make Whoopsie-Daisies Too!

Illithids have spent millennia intentionally evolving their reproductive cycle in the hopes of creating more powerful versions of themselves. They made giant, two-headed mind flayer bodyguards by infecting ettins. They created psionic amplifiers by turning beholders into mindwitnesses. They even figured out a way to make an elder brain mobile by fusing it to a tadpole-infected dragon. But all of these creations were made intentionally, with purpose, and are controllable.

Encephalons from Phandelver and Below: The Shattered Obelisk present a great gut-twisting option for DMs who want to add a suspenseful and thought-provoking wrinkle to their illithid-centered story.

Also, check your trunk, and make sure you haven’t left any eggs in there recently.

Kyle Shire (@kyleshire) is a contributing writer to D&D Beyond and a producer for Critical Role. In the past, he worked as a producer, writer, and host for Machinima Studios and Warner Brothers Interactive Entertainment. He's appeared on HyperRPG as the Mayor of Kollok and the Saving Throw Show. He currently lives in Los Angeles.