The importance of spiders...
by WingofCootSince the Chak planar spiders are major players, and Mygalla should also be very powerful magically, the intelligent arachnids (planar spiders and aranea) are going to be quite important here. And I'm thinking they should, mostly, be quite alien.
Mygalla should have both kinds of aranea- the X1-style standard monster (which I'm calling Webwalkers) and the Herath/Savage Coast type which has the ability to change between spider and human form (Shapewalkers).
Only Shapewalkers are by default suitable PC possibilities. Their shape-shifting ability is real, not just a disguise, and so they are able to directly experience human-style emotions and become fully and honestly members of human-style social groups. They have human brains and endocrine systems in human form.
Webwalkers (and Planar Spiders/Chak) are far more alien.
1. Arachnids are egg layers, and obviously don't produce milk like mammals. Parental investment in each individual offspring is far lower, offspring are independent far faster (they aren't born near-helpless like human infants), etc.
2. Arachnids are highly capable, venomous predators. An aranea is far more effective as a lone carnivore than any tiger. They are capable tool users, and work together well - but they don't need tools and cooperation to survive like humans do. Also, being pure predators, they can't use human-style agriculture. The nature of spider mouthparts means they can't eat things like grain/bread at all. Arachnid civilization is based on ranching of livestock (largely giant-insect livestock).
These two factors combined mean they're far less instinctively social than humans are.
They are highly intelligent, so of course they cooperate, and form societies and organizations. But because they are so self-sufficient, and because they don't form the strong emotional family bonds humans do*, their societies are organized along functional lines, and individuals don't have strong emotional ties to their society. The societies are more like human corporations than like tribes, kingdoms, nations, etc. - relationships of the individual with the group are based on benefit, and are quite changeable. An aranea changing loyalties isn't seen as treason - it's like going from working at Wal-Mart to working at McDonald's.
This means Mygalla is not "a nation" - though it is to the aranea's interest that the humans, Grens, Pyrithians, and other mammal-derived intelligent beings think so. It is simply the territory inhabited by the aranea, and is divided into many aranea societies. The changeability of loyalties + nomadic nature of Mygallan livestock raising means these societies don't have clearly defined borders.
Chak is more like a mega-corporation. It is strong not because of loyalty but because it has sufficient resources to ensure that its members contine to benefit from membership more than they would benefit from other options... so much so that it has outlasted any human-type empire or kingdom, dating back at least to the latter days of the Carnifex (c. 12000 BC) when the first Chak trading posts and watch posts were established in the Mystaran Solar System.
3. Another point is that they're very magically powerful. Webwalker aranea all have 3rd level MU spellcasting by default. Planar spiders are powerful spellcasters. That not only contributes to their self-sufficient nature, but along with their lack of traditional agriculture, means their civilization fundamentally doesn't look like a human one. Even a high-magic human civilization on Mystara like Glantri is still mostly based on non-magical farming. Alfheim, where everyone is at least a 1st level spellcaster, is closer- - and it's much less like a "normal civilization".
*note that they aren't emotionless - though human types often think so. They experience fear, anger, desire, etc. What they largely don't experience is the complex of socially-oriented emotions that human-types do: they can't be lonely, for example.