You see a name like “cave fisher,” and it doesn’t register with you at first, and as you study the stat block, you come to realize that the name is almost a perfectly literal description of what the creature that bears that name does: It sits in lightless caves, casts a line, waits for a meal to come to it, then reels it in.
A relative, perhaps, of the giant spider, the cave fisher is a human-size, wall-crawling arachnid with crablike claws. A solitary predator, with double proficiency in Stealth, the cave fisher is pretty dumb and inflexible, but it has one good trick up its sleeve: an extremely strong, extremely sticky filament that it can use to yank victims toward it from as much as 60 feet away. Coincidentally, I’m sure, 60 feet is the most common radius of darkvision, which means that if the cave fisher positions itself just right, it can lie in wait just beyond the ability of any prey, even prey with darkvision, to spot it. Even if it must lurk closer, expertise in Stealth plus disadvantage on targets’ Perception checks is a strong combo.
On top of that, it has Spider Climb, which allows it to move freely along walls and even across ceilings. This allows it to pull the dirtiest of tricks: hiding on the ceiling, then yanking its prey up to it, where if said prey manages to wriggle free of the cave fisher’s sticky strand, it has nowhere to go but straight down.
The cave fisher is a brute, with very high Strength and high Constitution, but even though it favors direct melee engagement, it would rather pull its foe to it than charge. Its Intelligence is extremely low, but its Wisdom is humanoid average: it has a normal self-preservation instinct, and it knows to target the old, the young, the weak and the oblivious. Regular readers familiar with my set phrases will ask, “What about the isolated?” Ah, but that’s the thing: Thanks to its Adhesive Filament, the cave fisher doesn’t need to target an isolated creature. When the creature it wants to eat gets stuck, the cave fisher isolates it—by reeling it in.
However, this ability has a weight limit: 200 pounds max. Many Medium-size humanoids, even if they themselves are svelte, will exceed this limit because of all the gear they carry. Also, the cave fisher’s ability to assess the capabilities of its prospective prey are quite poor. There are two ways you can determine which of multiple stuck creatures the cave fisher reels in with its Filament action. One is simply to say it judges on the basis of size: the smallest, weakest-looking target is the one it chooses. The other is to say it doesn’t choose, physics chooses: when the cave fisher uses its Filament action, the target with the lowest total mass is the one that stays stuck while all the others drop off.
Imputing any more sophisticated target assessment ability to the cave fisher would be a mistake, as would having it make obviously ridiculous choices like trying to reel in the dragonborn fighter in splint mail. (Armor is beyond its understanding, but the mass of armor isn’t.) In fact, it can probably tell from the force of a tug on its filament that an adhered creature is too heavy for it to reel in, and it won’t even try. It may be dumb, but its evolved instincts aren’t.
In reeling a target in, the cave fisher’s goal is to eat them, and that’s what it immediately goes about doing. Once it yanks an opponent up to it with the Filament action, it Multiattacks with its Claws until the target is dead—not simply unconscious, but dead. If a target breaks free, drops to the cavern floor and gets KO’d by the fall, the cave fisher throws out another filament and retrieves them, because, hey, free meal. This operation is not fast, however: extending the filament (described under the Adhesive Filament trait) and reeling it back in (described under the Filament action) are two separate actions that the cave fisher can’t take on the same turn. This fact is easy to miss, and I’m not entirely sure why the stat block was organized the way it is, because one might misread Adhesive Filament and fail to notice that it takes an action to extend, or conclude that because Filament is listed under Actions and Adhesive Filament isn’t, that the one action categorized as an action includes both extension and retraction.
Even so, the ceiling is still the best place for the cave fisher to wait for prey. Especially if its hiding place is 60 to 65 feet up, a fall from that height deals a lot more damage than the cave fisher can deal with an opportunity attack against an escapee who’s running away; plus, it’s a sure thing, while an opportunity attack may miss. That being said, however, if the choice is between a ground-level hiding place 60 feet from a traffic lane and a spot on the ceiling 30 feet above it, the cave fisher chooses the former, because the distance keeps it from being spotted by creatures with darkvision, and most subterranean critters in fifth-edition Dungeons & Dragons possess some means of seeing in the dark—if not darkvision, then blindsight or tremorsense.
Like other predators, cave fishers don’t like it when prey fights back, and they retreat when moderately wounded (reduced to 40 hp or fewer), using the Dodge action as they move away. They also skitter away if charged by multiple foes who have the ability to reach it, although the first thing they do is go straight up the wall to the ceiling.
I haven’t talked about their Flammable Blood yet, because, to be blunt, the Flammable Blood trait is goofy, and I don’t want to dignify it. It’s easy enough to say that it has no effect on their tactics, since (a) they’re too stupid to have any way of knowing that their blood is flammable, (b) they attempt to flee before they’re reduced to half their maximum hit points anyway, and (c) since they’re vulnerable to fire only after they’re that badly wounded, they’d have little opportunity to develop an instinctive fear of it—even more so since (d) they live in damp underground caves where they’re never going to encounter a natural open flame.
Per the flavor text, cave fishers are solitary by nature. Multiple cave fishers will be found in a single location only if they’ve been drawn there by especially good hunting, and when one catches a morsel, the others will try to snatch it away and consume it themselves. I suggest using only a single cave fisher if what you want is an actual combat encounter and using multiple cave fishers when you want them to be more of an obstacle—or if you want to mine the comic potential of having the halfling get helplessly yanked through the air by a bunch of albino spider-crawdads fighting over who gets to eat them while everyone else tries to figure out how to get them back down.