(From 5th Edition Monster Manual - 2014):

Survivors of the ancient aboleth empire, chuuls are crustaceans the aboleths modified and endowed with sentience. They follow the ingrained directives of their creators, as they have done since the dawn of time.

Primeval Relics. In the primeval ages, aboleths ruled a vast empire that spanned the oceans of the world. In those days, the aboleths used mighty magic and bent the minds of the nascent creatures of the mortal realm. However, they were bound to the water and could not enforce their will beyond it without servants. Therefore, they created chuuls.

Perfectly obedient, the chuuls collected sentient creatures and magic at the aboleths’ command. Chuuls were designed to endure the ages of the world, growing in size and strength as the eons passed. When the aboleths’ empire crumbled with the rise of the gods, the chuuls were cast adrift. However, these creatures continue to do what they did for the aboleths, slowly collecting humanoids, gathering treasure, amassing magic, and consolidating power.

Tireless Guardians. Chuul still guard the ruins of the ancient aboleth empire. They linger in silent observance of eons-old commands. Rumors and ancient maps sometimes lure treasure seekers to these ruins, but the reward for their boldness is death.

Whatever riches that the explorers bring with them adds to the hoard guarded by the chuuls. Chuuls can sense magic at a distance. This sense couples with an innate drive that leads them to slay explorers, take their gear, and bury it in secret locales aboleths dictated eons ago.

Waiting Servants. Although the aboleths’ ancient empire fell long ago, the psychic bonds between them and their created servants remain intact. Chuuls that come into contact with aboleths immediately assume their old roles. Such chuuls redirect their compulsions to the service of the aboleths’ sinister purposes.

(From 3.5e Monster Manual - 2003):

Like some large insect or monstrous crustacean, the creature rises from the still pool, its pincerlike claws snapping angrily as torchlight reflects off its mottled, armored carapace. Its small dark eyes fix you with a hungry stare, and the tentacles dripping from its mouth squirm excitedly as it emerges from the water. 

A horrible mix of crustacean, insect, and serpent, the chuul is an abomination that lurks submerged or partially submerged, awaiting intelligent prey to devour. 

Although amphibious, chuuls are not good swimmers and actually prefer to be on land or in very shallow water when they attack. They love to prey on lizardfolk. Chuuls are known to collect trophies from their kills. Although unable to use weapons, armor, or most other belongings, chuuls keep these items in their lairs.  If a victim has no interesting possessions, the chuul takes its skull. 

Most chuuls live in swamps and jungles, but some have adapted to subterranean life, hunting in and near underground streams and lakes. These underground varieties often prey on troglodytes and unwary drow. They are sometimes found as thralls of beholders or mind flayers. A chuul is about 8 feet long and weighs 650 pounds. 

Chuuls speak Common (or Undercommon, for the underground variety). 

Combat: A chuul prefers to wait by the shore, submerged in murky water, until it hears nearby prey (in or out of the water) that it can attack with surprise. 

A chuul grabs with its claws and constricts its foe, then passes the opponent to its paralytic tentacles. It tries to always have one claw free, so if it faces a large number of opponents, it drops a paralyzed or dead victim and continues attempting to grab, constrict, and paralyze the rest. 

Constrict (Ex): On a successful grapple check, a chuul deals 3d6+5 points of damage. 

Improved Grab (Ex): To use this ability, a chuul must hit with a claw attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can constrict or on its next turn transfer a grabbed opponent to its tentacles. 

Paralytic Tentacles (Ex): A chuul can transfer grabbed victims from a claw to its tentacles as a move action. The tentacles grapple with the same strength as the claw but deal no damage. However, they exude a paralytic secretion. Anyone held in the tentacles must succeed on a DC 19 Fortitude save each round on the chuul’s turn or be paralyzed for 6 rounds. The save DC is Constitution-based. While held in the tentacles, paralyzed or not, a victim automatically takes 1d8+2 points of damage each round from the creature’s mandibles. 

Amphibious (Ex): Although chuuls are aquatic, they can survive indefinitely on land. 

Skills: A chuul has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.