Another monster classic, the roper is a dungeon predator/scavenger that nabs its prey by camouflaging itself as a stalagmite or stalactite. The latter is rarer, probably because in every instance I can recall, the roper has always been depicted pointy side up; perhaps dungeon masters never consciously consider that ropers can also adhere to cave ceilings.
Ropers have enormous, toothy maws and sticky tentacles that lash out and seize their prey. Although their exceptional Strength and Constitution and below-average Dexterity suggest a brute fighter, ropers are ambush attackers, using their fast and flexible tendrils to compensate for their lack of mobility (their speed is only 10 feet per round, whether crawling or climbing).
Despite their low Dexterity, ropers have double proficiency in Stealth, along with the False Appearance feature, which allows it to blend in perfectly with its surroundings. I understand this to mean that passive Perception—and even Searching—will never reveal a roper for what it is as long as it’s holding still. Its Stealth skill comes into play only if it’s moving. Thus, a stationary roper will always take its opponents by surprise, as long as its eye is closed and its tendrils retracted until it strikes.
Usually I interpret Multiattack to allow attacks to be used in any order, but the roper’s is phrased in an unusually specific way: “The roper makes four attacks with its tendrils, uses Reel, and makes one attack with its bite.” The roper’s tendril attack doesn’t do any damage, whereas its bite does, so normally I’d suggest that a creature like the roper would keep making tentacle attacks until one hit, then reel in the target, then bite it, then finally use any tendril attacks it has left. But based on the phrasing of the roper’s Multiattack, I think the designers intended all the tendril attacks to be made first.
This limits the roper to a single attack sequence: tendril × 4, Reel, bite. The only variable is who gets attacked. Answering this question requires looking at the roper’s Wisdom—which is surprisingly high, actually. Although its Intelligence 7 indicates upper-bestial cognition and instinct-driven behavior, its Wisdom 16 suggests that it may be quite savvy when it comes to picking its battles—and its targets. But on what basis would it judge? A roper has no way to know that the adventurers traipsing through its cavern have capabilities that ordinary folks don’t. But it does know the difference between metal and flesh, and that it’s harder to bite through the former than the latter. So player characters wearing heavy armor may get a free pass at first.
A roper has six tendrils (unless one or more have been cut off). With a challenge rating of 5, a roper would have no reason to fear six of any kind of humanoid. Thus, with its four tendril attacks, it starts with the least armored opponent (disregarding Dexterity modifiers) within reach (50 feet!) and works its way up, and it uses its bite attack on the first opponent it grabs.
But note that the Reel action pulls a grappled creature only 25 feet. If the roper grabs its intended prey from 50 feet away, it won’t be able to bring it to its mouth in the same turn. So the roper doesn’t attack as soon as a group of foes comes within 50 feet. Instead, it waits for four of them (or all of them, if there are fewer than four) to come closer: within 20 to 25 feet.
ETA: Reader Novice DM makes an insightful observation: “You mentioned how ropers can be stalactites too . . . . As the roper grapples the party from up above, leaving them dangling in the air, the party must try to fight it either from afar or by getting up close. And there’s a danger to breaking its tendrils: you fall and take more damage, and you’re prone, so it’s harder to get away!” This is perfect. It’s exactly how flying monsters fight, so of course a climbing monster would do the same thing. In fact, it seems likely that a roper would fight from the floor only if the ceiling were more than 50 feet up. In all other instances, it fights from the ceiling. Not only does this make it harder for the roper’s prey to fight back, it makes it easier for the roper to get away if it must.
A PC who escapes the roper’s clutches and tries to run away will get a rude surprise when it gets 50 feet away, because as it leaves the roper’s reach, it will get an opportunity attack with one of its tendrils. Oops!
Incidentally, if one of the roper’s initial attacks misses, it will move on to the next target, unless and until it has to choose between an opponent that it’s attacked and missed and an opponent wearing a heavier type of armor.
If an encounter goes south for a roper, it’s in a lot of trouble: it’s much too slow to get away by Dashing or after Disengaging. For this reason, it’s willing to abandon a fight relatively quickly—after being only moderately wounded (reduced to 65 hp or fewer).
But it uses a tricky maneuver to do this, unique to itself: Since all its grappled opponents are also restrained and therefore have disadvantage on attacks and Strength checks (including Strength checks to escape its grapple), and since it has a very high armor class of 20, it can avoid being hit by opportunity attacks relatively easily. Therefore, while holding its grappled enemies in place—being grappled, they have a speed of 0—it takes either the Dash action or the Dodge action, depending on whether all its enemies are grappled (Dash) or one or more are free (Dodge), and moves away as quickly as it can, allowing its tendrils to play out. If it can climb up a wall while retreating, it does. Once it’s 50 feet away from a grappled enemy, it lets go of that enemy, until it’s released every one of them. This is its way of saying, “A’ight, fine, I’mma let you go, but I don’t want you coming after me.”
ETA: A roper that’s clinging to the ceiling, out of reach of enemies on the ground, also has the option of simply letting its prey fall.