What is an Oblex

The oblex is a new monster in fifth edition. The Dungeons and Dragons team created it with the help of Nolan Whale as part of the Make-A-Wish program. It’s a really nice story that you can read more about here (https://www.dndbeyond.com/posts/235-meet-d-ds-new-scariest-monster-in-mordenkainens). Published in Mordenkainen’s Tome of Foes, oblexes are slime monsters that eat memories. Once they crunch on someone’s memories, oblexes can impersonate them! These traits allow the oblex to perform complicated mind games. Players might not know that they are up against an oblex until partway through their confrontation with it.

Origin of the Oblex

Mind Flayers created oblexes in an attempt to create a slime that could hunt by attacking the minds of other creatures. oblexes hunt for the mind flayers by harvesting the minds of others. Presumably, this means that mind flayers are able to extract the memories that oblexes feed on.

Oblexes are born as oblex spawn, which are small, green globs of slime. As oblex spawn eat more memories and assault more minds they take on the personalities of the people they are draining, only more twisted and cruel. The more memories they eat, the larger they grow, and the more red in tone they become. As they become bloated with memories, oblexes begin to go insane. When this happens, oblexes release some memories to relieve themselves, creating a new oblex in the process.

Abilities

As oblexes get larger, they gain the ability to create imitations of people whose memories they have eaten. They can create up to five copies of creatures called simulacrums. A small trial of slime that reaches up to 120 feet connects simulacrums to the oblex. The simulacrum is identical to the creature it is imitating except that it smells of sulfur. Oblex simulacrums are an extension of the oblex, allowing the oblex to exist in multiple spaces and use its abilities without revealing itself.

Adult oblexes have a limited selection of innate spells to choose from. These spells include charm person, color spray, detect thoughts and hold person. Oblexes use these spells to control enemies while eating their memories. Mechanically, the oblex eating someone’s memories deals damage and requires the target to subtract a d4 from all of their attack rolls and ability checks. As the oblex continues draining memories from the same target, the d4 increases by a step, all the way up to a d20. If the oblex drains the target again after that, they fall unconscious. In my own game, I would upgrade this to kill the target once the die exceeds a d20. This would mean seven failed saves, so adding that extra punch to the ability is fine.

An elder oblex is a more challenging version of the adult oblex with a few more spellcasting options. They gain more crowd control spells like Hypnotic pattern and fear. They also learn a defensive spell in the form dimension door. The most interesting spell they gain is hallucinatory terrain. In combination with the ability to create simulacrums, illusory terrain allows oblexes to create complex deceptions to lure their targets into danger.

All oblexes share a weakness to fire. Anytime they take fire damage they have disadvantage on attack rolls and ability checks for a turn. Despite this, oblexes do not have a vulnerability to fire, so they do not take additional damage from fire spells.

Encounter/Adventure Ideas

In The Underdark

Because mind flayers created oblexes, oblex spawn can make a good monster in the Underdark to hint that mind flayers are around. Additionally, oblexes have a good chance to fool the party and lure them into a trap in the harsh Underdark. Using hallucinatory terrain and their simulacrums, oblexes can create the facade of a helpful settlement. They can provide aid and a place to sleep for the party, with the goal of devouring their memories in the night. The party might be suspicious of this, but that’s half the fun. If they don’t discover the ruse right away, I would have each one wake up under the effects of the oblexes eating their memories. This will definitely get players worried and wanting to figure out what’s going on.

Another adventure could be exploring what seems like an abandoned mind flayer lair. As the player characters travel through the structure, they can run across dead mind flayers. On the way through the chambers, they can have combat encounters with simulacrums of mind flayers and small oblex spawn. When they reach the center of the lair, they’ll find an extremely bloated oblex that has been feeding on the elder brains memories. I would use a beefed-up version of the elder oblex for this encounter, adding a little hp and ac and a few spells that deal psychic damage.

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In the World

Although mind flayers created oblexes, oblexes are still able to travel above ground. If a DM wanted to they could easily change the origin of the oblex to something unrelated to the underdark. Removing their tie to the underdark makes the oblex a more versatile monster.

Oblexes can fill the same role as doppelgangers in that they can mimic targets. The skillset of an oblex allows it to take part in a more grand illusion. I’ve always loved the idea of a group of oblexes slowly taking over a town. If the players frequent a town they might notice its citizens getting a little more mean or cold. Despite this hint, it is unlikely that they would think much of it before the oblexes could eat the memories of many people and reproduce. At this point, it would be near impossible for players to know what characters they can trust. When the players sleep in town you could even have one of their memories get eaten, allowing the oblex to create simulacrums of them. As long as the player is into the idea you could even make the party question if they can trust each other.

If you want to take it a step further, you can make a complete town of imposters. All the citizens could be oblex or doppelgangers working together to draw prey to themselves. You could even make the buildings huge mimics if you want to lean into the imposter theme.