A. Too many legs, they move weird
B. Webs in your face ack
C. Clearly cleverer than other bugs
D. Other stuff like poison and reading over your shoulder when you browse the internet
We can actually have them move strangely within the game's movement system. Spiders get to move after every player action. They sense who is next in the initiative order, via spider-sense, and even what they are planning to do and so try to move somewhere disadvantageous for them. Anyone who has thrown a shoe at a spider can attest to this power.
If a player does nothing, the spider doesn't move after their inaction. Anyone who has had a silent staring match with a spider can attest to the truth of this.
Now lets talk about spider webs. Turns out they aren't flammable like that old D&D example encounter describes. The proteins just denature and coagulate. Spider webs melt, so a torch is handy to escape with, but you can't toast a nest with a single match.
Also, spiders aren't immune to their own sticky webs, they just have maps of where the safe silk to walk on is. Spiders can totally be owned by their own webs if the fighter throws them into the wrong spot. This means that you can make spider-web mazes that are actually puzzles and not just deathtraps. And other neat stuff too. There are spiders that weave scuba gear for themselves, spiders that weave bolas, spiders that weave doors, all sorts of stuff. Also, they are designed to catch stuff with mandibles and razor exoskeletons. Just cuz you have a sword doesn't mean you can cut your way out if you get wrapped up. Maybe a magic sword, but if this is a spider that big, it's probably a little magic itself too, so a generically magic sword probably won't cut it. If you get totally caught up, you can't rescue yourself. That's a game over. Like being petrified.
STAGES OF STICKINESS
1.After the first failed save, Half movement. If you stop moving, you may be able to carefully extricate yourself. It doesn't matter what you try to do, the web design has anticipated 90% of all possible prey-actions and whether you charge forward, flee backwards, or stand there flailing around in combat, you'll have to make a save each round or get more webbed up
2. Walking impossible, hindered attacks and actions. Disadvantage or a bane or whatever. This probably takes like, a minute or more to escape from. Or fire to melt the strands.
3. Following the 'three saves and you're out' rule, you're all tied up and all you can do is wriggle. You'd need a long, long time to escape and will definitely be eaten before you manage unless you have a really good trick up your sleeve.
Now we get to cleverosity. Spiders have 8 eyes, one for each side of the d8 HD. This means spiders can tell how many HP things have, and so they can tell when they should or should not pick a fight. This also means if you communicate with spiders, you could keep a friendly one around and ask it what the powerlevel of your enemies is. Also they have websense and all that but you knew that already.
And culturally, spiders have ok odds of being friendly and intelligent. Bad spiders tend to poison people with their mere presence and also have tricksy abilities, but good spiders tend to be wise and full of secrets. Mythologically speaking, they're similar to snakes- clearly not man's best friend, but not man's worst enemy either.
In any case, if you have spiders be pretty much just a tiger with a poison bite, or functionally identical to a bunch of swarming poison centipedes, you're not doing spiders justice. And by the way I've totally used spiders lamely to no effect so I'm speaking from experience. If you want to translate the spookiness of a spider to tabletop, you hafta go the extra distance.
So here's some approaches, that all feel like they're from different genres.
Leper Spider-Poison Spider- If the poison does 1d6 con damage and you get two saves, of course no one is gonna be scared. If the poison is save or instadie, there's not much the players can do but shrug and carry on.
The happy middle ground is of course, horrible maiming.
These spiders cause intense, chronic vasoconstriction. Afflicted limbs lose bloodflow, turn white, and rot. Amputation is necessary or the victim will probably die of gangrene. You get three saves- the first is to see if the spider actually bothered injecting venom or if it was a warning bite (known in the business as a 'dry bite'.)
The second save is to see if your limb loses functionality over the course of the next minute and stays that way for a few days. It's pale and weak from no blood and you can barely move it. If you make your save it's just debilitated for an hour or so.
The final save is vs gangrene as your flesh necrotizes. You can choose amputation and avoid having to make this save, but if you fail this save, you die of gangrene as you stubbornly cling to your rotting limb. At this stage cure poison can't help you because the poison wore off as the rot set in.
These spiders can be a swarm, a single coin sized one that has descended to your backpack on silken thread for inscrutable reasons, or varying degrees of bigger and bigger. They are more hunters than lurkers and only use thread for climbing.
Leper spider venom, if properly applied, can be really good for disguising yourself as, well, a leper.
Jorōgumo-I think every animal in Japan has a myth where it turned into a cute girl and ate somebody. No exceptions.
In any case, this probably won't end well. The adventurers might appreciate free gifts from their mysterious fan, but probably will fail to reciprocate feelings, and so the spider will feel hurt and betrayed. Depending on how far things get, the players may be hounded by divorce lawyers, or pressured into a wedding by the spider's burly parent. Depending on how magical the spider is, breaking a promise made could result in the character being whisked away by an invisible dragline thread. Or maybe the spider will get increasingly desperate and become a jealous stalker out to sabotage all other romantic prospects. Maybe the spider will sell their soul to demons for a hot human body and try to guilt the character with the ol' 'I sold myself into eternal damnation because I love you' line.
Or maybe the player will decide to go along with it. Before they can retire in marital bliss though, they probably need to find a polymorph spell, and definitely need to find 8 wedding rings. Comedy, tragedy, drama, all sorts of fun could be had. It might even be spooky as the players investigate and find out this adoring fan is something weird and uncanny.
Fiery Spiders- The Choctaw say Grandmother Spider stole fire and presented it to animals, but animals refused it and so she gave it to humans. Jorōgumo are also sometimes depicted as controlling fire-breathing spiders. Fire sometimes gets used as an 'upgrade' from poisons, as you see with dragons. Old dragons tend to be poisonous, while it's the newer ones that breathe fire. Anyway, poison and fire as spreading, hard to control dangers makes sense.
But just making the spider breathe fire is no good. That turns our encounter from a spider encounter to essentially a 8 legged dragon encounter. Suppose it's the silk- it looks normal, but you feel heat radiating from it. It sears wood and scalds flesh. Naturally it can't be burned, and structurally a burning web is more barrier than snare. Drifting strands turn a hallway deadly, a wispy web stretched across a doorway is like a wall of fire. The Spider could leave silk behind it as it scurries around, a scorching tripwire. This is a web that works like a burning house- still with themes of entrapment and building danger though. The venom burns, of course. This sort of scenario is less horror and more terror though. Similar to a dragon attack, perhaps. Maybe in place of. The players set out to hunt a fiery dragon, find a cave strewn with burning webs instead, and a monster that is trying to lure the players in, rather than one that's trying to ward the players away.
Wise Spiders and Wiseass Spiders
Sure there's sagely spiders like Grandmother Spider, but there's also tricksy spiders who use their wits to get up to all sorts of shenanigans like Anansi. If a spider steals something or other, all the powers of a spider could be put into escape rather than entrapment. Spiders could be master thieves (or assassins), all they need is a bit of motivation, and then a pursuit is on!
Spider steals priceless diamond as players enter room, retreats beneath floorboards. Shenanigans ensue. It writes itself, really.
Cosmic Spiders And no I definitely don't mean in the 'has 2 million HD and eats neutronium golems' way. Spider webs look like they're strewn with stars when dew is on them and the light is right. No doubt they're chock full of secret cosmic wisdom, but if you don't have time to cajole and bargain for the info, you can divine it yourself with the proper ritual.
This spell alters a wizard's thinking so that they may understand the secret clues hidden in spider webs.
The targeted web...
- contains a dead fly
- contains partial spell information. The spider in question can probably cast it.
- contains a juicy secret, ranging from gossip to forbidden lore
- is a treasure map. The spider deserves a 5% share if you're being fair and just.
- is a prophecy telling of events yet to come.
- acts as a dreamcatcher. It may have an errant nightmare trapped within.