Believe it or not, until I cracked open the fifth-edition Monster Manual, I’d never heard of chuuls. I got into role-playing games with Advanced Dungeons and Dragons, back when God’s grandma was a little girl, and we didn’t have chuuls back then, not even in the Flumph Folio. According to the all-knowing Wikipedia, chuuls were introduced in the third edition. (Maybe not so all-knowing: it describes them as “extremely intelligent,” while according to the 5E MM, they have Intelligence 5.)
Chuuls are, more or less, enormous, semi-uplifted crayfish, servants of the mighty, ancient aboleths. They’re amphibious, chosen for their role because of their ability to survive on land as well as in the water. They’re larger than human-size and exceptionally strong and tough, predisposing them to be brute fighters. Although they’re not exceptionally dexterous, their chitin gives them an armor class of 16. They can sense magic, and we can infer from the flavor text that they’re drawn to it, obeying an ancient, instinctual command to gather powerful items for the aboleths that ruled them.
Chuuls have darkvision, suggesting that they move about on open land only at night and spend the rest of their time either underground or underwater. Judging from the flavor text, they don’t seem to have a lot of motivation to go wandering around but rather will stick close to locations that they feel some urge or duty to guard. They aren’t conscious of any such duty, however: with Intelligence 5, they operate strictly by instinct.
They’re also not particularly complicated. As brute fighters, indiscriminate in target selection, they’ll generally move to attack the first enemy they sense. The range of their senses, however, varies: their darkvision allows them to see another creature out to a range of 60 feet, but their Sense Magic feature extends to 120 feet, so the presence of a spellcaster or the possession of magic items in an adventuring party will catch their attention. And they’re highly proficient in Perception, so good luck sneaking by.
The chuul’s basic attack action is a Multiattack comprising two pincer attacks, plus one tentacle attack if it has a creature grappled. The grappled condition comes from the pincer attack: any hit with a pincer is an automatic grapple, from which a target can try to escape on its own turn.
Despite being a brute fighter, and thus generally unconcerned about taking hits from a melee opponent, the chuul has a 10-foot reach, so it doesn’t need to get any closer before it attacks. Consequently, when it moves to attack, it will stop moving as soon as it’s within its own reach of its target.
The Multiattack action doesn’t specify the order of the chuul’s attacks: they can go pincer-pincer, pincer-pincer-tentacle, pincer-tentacle-pincer or (if it’s the second round or later and the chuul has an enemy grappled already) tentacle-pincer-pincer. The tentacle attack does no damage but injects a paralyzing poison. This poison prevents a victim who fails his or her saving throw from taking actions or reactions, causes him or her to automatically fail Strength and Dexterity saving throws, grants advantage on attack rolls against him or her and turns every hit into a critical hit as long as the attacker is within 5 feet. It lasts 1 minute but wears off sooner if the victim makes a DC 13 Constitution saving throw—which he or she probably will at the end of his or her first turn after being poisoned, so the chuul doesn’t have a lot of time to take advantage of this state.
Thus, a chuul’s first action will be a Multiattack, starting with a pincer. Every pincer attack that misses will be followed by another pincer attack. Every pincer attack that hits will be followed by a tentacle attack. Every tentacle attack will be followed by a pincer attack—but not necessarily against the same opponent. If a chuul has an enemy grappled but is unsuccessful at poisoning him or her, it will direct this next pincer attack at someone else, aiming to grapple a second victim. On the other hand, if a chuul does have a poisoned victim in its pincer, it will keep pincer-attacking that victim.
Let’s say the chuul’s first attack is against Berach the sorcerer. The first attack is always a pincer attack, and it hits, grabbing the elf and pulling him closer. The second attack, since the first was successful, is a tentacle attack. Berach, unfortunately, is a fragile flower—his player chose Constitution as his dump stat—and he fails his saving throw. He’s poisoned, paralyzed and incapacitated. The third attack is another pincer attack, against Berach, with advantage, and when it hits, it’s an automatic crit. Owww. Poor Berach is down already.
In the next round, no one else smells like magic quite as much as Berach did, but the sword that Guthrún the barbarian is swinging sure does. The first attack is a pincer attack; it hits, and Guthrún is grappled. The second attack, since the first was successful, is a tentacle attack, but Guthrún has Constitution 18 (her player rolled her stats instead of buying them with points) and is unaffected. What now?
Usually, in the 5E MM, when a monster (such as the recently examined kraken) has an attack that automatically grapples on a hit, the grappled target is also restrained, but that’s not the case with the chuul. Since it has no advantage on attacks against an unpoisoned, unparalyzed, fully capacitated barbarian, rather than attack her again, it turns its attention to Sir Amalric the paladin and aims a pincer attack at him. Amalric is wearing chain mail, though, and the chuul’s attack fails—it can’t get a grip on him.
Between the second and third round, Guthrún breaks free of the chuul’s pincer. When it tries to attack her again, it misses. Meanwhile, also between rounds, “Three-Cheeses” Jack, the stout halfling rogue, comes around and tries to hamstring it with a magic dagger, drawing the chuul’s attention away from Amalric, so its next pincer attack is aimed at him. It gets him, and now the chuul can use its tentacle attack on him. Three-Cheeses resists the poison as well, but he’s not strong enough to break the grapple on his own turn.
Thus, at the start of the fourth round, while the chuul holds Three-Cheeses in its claw, it uses its tentacle attack to try again to poison him, fails again, then aims its next pincer attack not at Three-Cheeses but at Guthrún again. See the pattern? The chuul only pincer-attacks a grappled target if that target is poisoned. The chuul can tell whether or not Three-Cheeses is poisoned, but it’s not smart enough to figure out that if the poison didn’t work on him the first time, it probably won’t work on him the second or third time, either. So it gets stuck in a loop, attacking other targets with its pincer attacks yet trying again and again to poison Three-Cheeses with its tentacles. Meanwhile, Three-Cheeses, grappled but not restrained, can aim attacks of his own at the chuul without disadvantage or any other penalty.
Chuuls do have a flight instinct that kicks in when they’re seriously injured (reduced to 37 hp or fewer). Lacking the intelligence to Disengage, they Dodge as they retreat toward the nearest body of water, then switch to the Dash action as they swim away.