Octaros are a race of diminutive, subterranean goblinoids, each about 4' tall. They have eight fingers on each hand and eight toes on each foot. They each have 1 eye. They are born, live, and (usually) die in groups of eight. The octaros call these sibling groups families. To them, parents and offspring are not family. Only your seven other brothers and sisters are family.
Or actually brothers and sister. Each octaro family is made up of one female and seven males.
Octaros have fleshy faces and lips, but no external ears. Their teeth aren't individuated, but are instead fused together in a bony ridge of needles. These teeth-arcs make natural bracelets, and are frequently used as such by the octaros themselves.
They have no bones, and are instead supported by cartilage. This makes them weak and wobbly--they nearly fall over every time they attempt to run. However, this flexibility is sometimes a boon--octaros can usually recover when they are crushed by cave-ins (and dug out by their family). Combined with their small size, they can easily survive long falls. Combined with their grippy hands and excellent climbing ability, they are well-suited for the wet caves in which they live.
They live in squalid villages, and wear rags. They have an inexplicable attraction for fish skeletons, and use them to ornament everything imaginable. Although the female will only mate with males from outside her family, they largely shun normal relationships outside the family (no marriages, no institutions, no inter-family friends).
Instead, all of their social functions take place within the family. Octaros are constantly babbling and chattering amongst themselves. They have a complex language, and when one octaro family speaks to another octaro family, they all speak simultaneously. Although this language isn't fully understood, it seems that full expression of it requires several simultaneous speakers.
In personality, they are curious and shy. They react well to gifts of shiny objects, spears, or interesting fish bones. When they are fond of you (always a transient emotion) they will all give you a simultaneous hug. They are also utterly amoral, and will not hesitate to kill and eat a traveler if they think they can get away with it. However, their favor and displeasure is short-lived, and by the next day their attitude towards you will have "reset". Short memories, I guess.
Like many of the deep subterranean races, they worship some alien pseudo-deity called "The Watcher Behind the World". Despite the ominous name, this deity is supposed to reside in the deepest trenches in the blackest ocean. They sacrifice fish to it, but not sentient creatures. Honestly, compared to other chaotic gods of madness and void, the Watcher is downright friendly.
This is where it gets weird. In addition to the way they constantly talk to one another, they also seem to have low-level telepathy. If one octaro is startled by something, the whole family all jump simultaneously. If one is hurt, they all seem to feel pain. If one is told something, the others will soon learn it too, even when they are prevented from talking to each other.
It goes deeper.
If one octaro is starved while the rest of his family is fed, the starving octaro will not die. He may grow a bit skinnier, but his overall health is remarkable. Similarly, as long as most of an octaro family can breath, the ones who cannot breathe will not drown or suffocate. (They use this ability to survive cave-ins, as well as to forage for food on the bottom of subterranean lakes.)
If several octaros of a family are killed, the remaining ones (who shared the pain) will also die. Experiments have shown that this number is usually 3-5 of the octaros. When one is infected with a disease, the others will soon share it. All of these things are true even when the family is separated by lead walls.
A family can even survive with 2-3 of its members missing, although this causes them tremendous psychological trauma. It's almost enough to make you feel bad for the little hellspawn.
Among the noble wizards who have investigated these little beasts in the name of enlightenment, the reigning consensus is that octaros are an offshoot of goblins that have developed an intense psychic link. A supernatural degree of empathy, if you will. So it should come as no surprise that octaros have some minor psychic powers.
Octaros can telekinetically lift one another. It's not a true telekinesis; the opposing force is distributed across the rest of the family. If a 70 lb octaro is telekinetically lifted off the ground, the remaining seven will each weigh an additional 10 lbs. They can even lift a dead family member this way. Likewise, if an octaro's family use their telekinesis to pull a family member back out of a fire, they will be pulled forward a slight amount. Momentum is conserved.
Attempts to teach octaros to harness their psychic powers more broadly have failed, even under the threat of the most severe punishments. The octaros claim that they are unable to lift anything except for members of their own family. If this is true, it's the first instance where telekinesis has manifested in such a limited extent.
Octaros cannot be separated more than 30' from their family. If the removal of one octaro is attempted, the remaining octaros will restrain it. It's as if there is an invisible rope tied between them. However, if you pull hard enough, the psychic rope will rip, and the separated octaro will spasm, vomit blood, and die, presumably all symptoms of the psychic trauma it endured. The remaining octaros will be very upset, of course.
They literally cannot live without each other. It's an absolutely unprecedented level of sibling co-dependence.
Treat them as goblins that always travel in packs of 8, but with the following changes.
Octaros cannot be separated more than 30' from their family members (at least, not without superhuman strength or a good block and tackle). This persists into death, for at least several months.
Octaros can "telekinetically grab" each other. This prevents them from falling into holes or being tripped (unless it happens to the whole group) as well as reach things that are 30' up on the ceiling.
They share thoughts. One isn't surprised unless they all are. Mind-affecting effects (like charm) have no effect unless they affect the whole family.
If one is poisoned, they all are.
A family of octaros dies when 3-5 of them are killed. Three (25% chance), Four (50% chance), or Five (25% chance).
All those wizards in the previous paragraphs are wrong of course.
Bags of holding have a dark twin known as bags of devouring. These appear to be normal bags of holding, but when objects are placed inside it, the objects are devoured, presumably to be swallowed into some connected, extradimensional organism. The bags of devouring are just mouths in our world that connect to some greater creature outside it.
Octaros are the same way. Each is the "tentacle" for an "octopus" that exists outside of our dimension. Through this central body they share nourishment and thought. Each octaro is a fingerpuppet on the same hand. It's curious that octaro families seem to have forgotten that they are part of a singular organism, and have taken to imitating the cultures of other underdark civilizations, especially goblins. (Does their mind reside in each octaro, or in the central, unseen mass?) Their constant chattering to each other might have its roots in imitation, or it may be a way to sooth a ragged soul.
Even more troubling are their old tales. They speak of the old cities, buried deeper down. So deep in the earth that fire runs through the walls. Where octaros lead armies as slave-takers and master lobotomists. Levitating shieldbearers lead the armies, while the ranks are filled out by larger beasts--carnivores--of a similar physiology.