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Have you ever wondered why the rations you put in your bag of holding sometimes disappear?
The humble bag spider is large by the standards of mundane spiders, with mature specimens being the size of a tarantula. It is, however, not a very accomplished hunter. Its web-spinning abilities are all but vestigial, its poisonous bite incapable of causing anything more serious than a mild rash, and it lacks the speed and defenses of other large spiders. For all these weaknesses, however, it has one unique ability.
A bag spider will seek out a hole, preferably one very close to their own size. In nature they will choose a hole in a tree trunk, a crevice in the rocks, or a similar opening. They can dig holes to use as burrows, although they are poor diggers. Then, it will start expanding its hole. It will bite and gnaw at the sides, spinning webs along the walls as they expand their burrow. The space inside their hole increases, but the outside remains untouched. As part of this process it is rendered airless, although much like the diving bell spider, the bag spider has its own method of gathering up bubbles of air and storing them in its burrow. In this way, the bag spider creates its own perfectly safe demiplane, where neither its prey nor its predators can easily survive.
In nature, bag spiders were never all that successful. While their little 'nests of holding' rendered them incredibly safe, they were poor hunters. And when they grew too large for the entrance to their burrow they had to find a larger one - a journey that was often fatal for the bag spider. But that all changed with the arrival of humans.
Bags, pouches and pockets make perfect bag spider nests. They're common, easily accessible, perfectly camouflaged, and humans sometimes put food inside of them. In fact, when a bag spider turns a bag into their nest, humans will often continue to place food inside of it. The bag spider, meanwhile, hides from the giant apes - they are far too large for it to hunt effectively, and capable of crushing it with a single blow. This means that a bag spider might go completely unnoticed, save for the occasional disappearing rations - few people explore all the dark corners of their bag of holding after all.
Of course, not all bags of holding were created by bag spiders. Wizards were happy to copy the concept. But if the right pocket of your old pants is suddenly strangely roomy, or the saddle bags of your donkey seem bigger on the inside, chances are you've got a bag spider infestation.
How to use Bag Spiders
Your players will love having a bag spider pet. The smart but shy spider can be taught tricks, fed bacon bits, and live in their bag of holding. Over time it will grow to the size of a dog, and will require larger and larger bags. Maybe they end up keeping it in a barrel. Maybe it lives in their saddle bags. It will happily bite the fingers of any prospective pickpocket, and its presence will increase the bag's holding capacity by 50% - it nicely organizes everything, and maintains the magic, after all.
Your players will be baffled to find one of their pockets, bags, boxes, barrels, pouches, or hats has suddenly become bigger on the inside. And they'll be shocked when the spider nips out and eats some of their rations while they're sleeping. Will they kill the spider, or befriend it?
A truly massive bag spider could have a nest the size of a house, containing an airless domain many times that house's real size. It might steal chickens and sheep at night - the only things dumb and slow enough for it to reliably hunt. But how are the players going to hunt a giant spider in its own domain, where they can't even breathe?
If you really hate your players, introduce Parasitic bag spiders. They live in your nose, or ears, or mouth. They use poison to leave that area of your face numb, and increase the size on the inside, creating a space for them to live. They're smaller, the size of house spiders, but hundreds of them could be living in your ears. And you can hear them chewing at night....
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