A three-eyed fish might not seem like a major threat to veteran Dungeons & Dragons players, but the aboleth is actually a terrifying threat that can shape an entire campaign. For months, I've terrorized my Dungeons & Dragons players with an unusual monster - the aboleth, a large three-eyed fish with minor psionic abilities whose existence pre-dates the gods themselves. The aboleth dates back to the early 1980s and is usually used as a remnant of times past. While aboleths once ruled over the world, they typically can only be found in underground pools in remote corners of the Underdark. In combat, the aboleth's abilities are limited outside of the water - their main abilities are the ability to forcibly enslave up to three creatures per day and to infect those who attack it in melee range with a disease that temporarily turns the victim into a water-breathing creature.
So, how did a mucus-covered fish turn into a major villain that plagued my players for months? The first step was to provide the players with an intimate link to the aboleth, one that gave them a reason to actively want to stop the creature. In this case, the players accidentally unleashed the aboleth from his ancient prison while searching for a lost treasure - a Perfect Conch sacred to the region's Crab Folk. The initial encounter ended in a draw after the aboleth enslaved one of the party members - the aboleth agreed to let the mind controlled character go, but only if the players walked away from the fight.
Once I established the connection between the party and the aboleth, the next step was to turn the aboleth into a potentially cataclysmic threat. Traditionally, aboleths are schemers - they seduce minions in by promising their greatest desire and their enslave ability works best when others don't know that there's a puppetmaster manipulating them behind the scenes. Because the players knew about the aboleth from the onset, I choose to lean into the mind control aspects of the aboleth instead. The aboleth quickly became a powerful force working in the background of the campaign setting, using chuul minions to kidnap innocents to turn into pawns for his growing army. While enslaved creatures typically can attempt to shake out of their enslavement whenever they take damage, I came up with an in-world reason for why the aboleth could maintain control of so many people at once without fear of revolt.
While the players had few direct encounters with the aboleth, they spent months fighting its chuul minions, discovering mind controlled puppets wandering through the woods at random, and then suffering periodic psychic migraines caused by the aboleth's continuous psychic assault on the region. They saw the aftermath of the aboleth's schemes from time to time, discovering battlefields filled with dead chuul and other creatures, and even learning that previously friendly factions had either willingly or unwillingly joined the aboleth's cause. They met creatures fleeing from the aboleth, including the Crab Folk who were honor-bound to follow the creature who held their Perfect Conch artifact.
Once the players decided to confront the aboleth directly, I put them on a multi-part quest designed to "even the odds" and bring the aboleth back down to the player's levels. The players had to figure out a way to disrupt the aboleth's enhanced psionic powers, while also protecting a handful of powerful NPCs who were being targeted by the aboleth directly. Eventually, the players obtained a handful of special crystal weapons that would cut off the aboleth's mind control and ambushed the aboleth at his above-ground base, an under construction artificial lake that would provide the aboleth with a permanent place of power.
The final fight was a tense encounter - the players successfully ambushed the aboleth and got a valuable surprise round, but over half of the party missed on their initial attacks, which kept the aboleth's enhanced psychic powers in play. Because the players chose to hit the aboleth with ranged attacks, they also didn't have enough melee combatants who could actually try to get the Perfect Conch away from the aboleth. Because they couldn't get the Perfect Conch away from the aboleth, they were nearly overwhelmed by the combined Chuul/Crab Folk forces. In the end, the players managed to score a killing blow on the aboleth, but not before the party's Rogue was ripped to shreds by the once-friendly Crab Folk.
While my players ended the threat poised by the aboleth, this isn't necessarily the end of the creature's story. When aboleths are killed in the Material Plane, they don't permanently die. Instead their spirit returns to the Elemental Plane of Water, where they wait for a new opening to return to our world and begin a new scheme. Let's just hope this particular aboleth doesn't return during my player's lifetime.