A Primer on the People

Who lives in the Valley? The place is wet 90% of the year, and is filled with death cults, angry undead, and violent wildlife. It sounds like the question should be:
==== Why do people live in the Valley? ====
Because it's home. Why do people live anywhere? The ground is fertile, the views are nice. The Valley is more than just a place, it's a state of mind. It means persevering on hot autumn days. It means seeing time stoats and dimension deer and more strange things in a week than most people will see in a lifetime. It means knowing that not only are the gods watching you, they know you by name.
==== The gods don't just watch ====
The Gods of the Valley (both those 'native' to the place, and greater gods who watch over much larger areas) are famous for being interventionist. No matter their place on the alignment grid, the gods can't help but interfere in the lives of mortals. Orcus (the demon prince of the Undead) is willing to grant great power to mortals at the drop of a hat, just to see what they'll do with it. Mitra, god* of compassion, will often send angels and other divine creatures to inspire the masses.
This doesn't mean the gods are any more human or less inscrutable. They're powerful, alien beings with complicated agendas.
*"God" is gender neutral. With a few exceptions, all gods take gender neutral pronouns.
==== What if you're not a god or a hero? ====
In that case you farm, hunt or trade. Farming is simple; the Valley is fertile and home to some crops found nowhere else. Even with the impressive amount of magic available in the Valley, there's nowhere enough to feed everyone. Grains are the most popular crop; mostly barely and wheat, as well as a Valley-native grain called ricegrass. Grazing is highly profitable, but difficult due to the aggressive wildlife. 
Hunting is probably the most common profession, although it's mostly concentrated in the 'uncivilized' areas of the Valley controlled by orc, goblin and gnoll tribes. These creatures can exist with a fully carnivorous diet (occasionally supplemented with wild berries or mushrooms). The most common prey are boar, giant spiders, dire-rats and deer, as these creatures are the best calorie-to-violence ratio. Other creatures like dimension-deer, dopplebears and ultramice are more trouble than they're worth. 
Trading is a surprisingly reliable way to make money. Even with magical aid, moving tonnes of wood, stone and grain around is difficult. With prices set in advance by the company in charge of managing trade (Ironclad Goods and Services), a trader can be sure that their wagon full of barrels of wine will turn a decent profit for two weeks of bumpy roads.
==== Wait. "Ironclad Goods and Services". Is this one of those "Great Companies" you talked about? ====
Yes
==== Are you going to talk about them? ====
Yeah, next time
==== You said this primer would only be a 3-part series ====
I lied to you.