You have learned much in your four years underground. These are the tidbits you may have learned. Whether you actually know them I will leave to your whims and background (except for Shadows and Plots, see that section for whether your PC can know anything about that.)
There are no great nations or peoples in the underworld, just small groups scratching out a living. The Dwaro are a common people who have lived below ground since before the Titan fell. They are short, generally under 5 feet, pale, slender and unbearded (i.e., not your stereotypical Tolkien or D&D dwarf; more like a Norse Svartelf). Their language is the common language of the underworld and one that any of the PCs can have learned during the sojourn if they choose (see Paragon Leveling). Some Dwaro possess traces of their old magic but most a weak relying on stealth, guile and thievery to survive.
There are odd-humans, elves and other races in the underworld, some who have lived below ground for millennia but these rarely form single race groups and instead live among the other races. The exception are some elves who claim to trace their lineage in the underworld to the Dawn Age. These elves are of slighter build than their surface cousins, often but not always, with ebony skin and white hair. Despite lurid rumors on the surface of an affinity to spiders and bizarre half-spider, half-elf monstrosities, none of the PCs have seen or heard credible rumor of such things. Regardless, the drow as these dark elves call themselves do sometimes make their own settlements and are often hostile to outsiders.
The other significant race in the underworld are tieflings. Tieflings are demon tainted creatures. In the underworld they are typically Dwaro who were tainted but a tiefling can come from any race. Recall that demons were created during the Kazelthume empire when the Dwaro delved deep in the High Montains and found a being who corrupted their people over centuries, ultimately leading to the loss of their seven greatest cities. Some corrupted dwaro became the ancestors of the demons and devils, including Lubgrubbers, but in other cases, the taint is more subtle and manifests itself as tiefling traits.
On the surface, tiefling traits are rare and often skip many generations. In the underworld, tieflings often breed with other tieflings and there whole communities of tieflings. In time, some such groups grow more demon-like but usually they remain stable in terms of features and degree of taint.
Due to their affinity to true demons, tieflings are distrusted and often keep to themselves but some of the larger settlements do have tiefling communities.
The Dwaro typically live in small settlements of a few extended families and 20-60 individuals. Such settlements are transient, often lasting only a few years, but the Dwaro are quick delvers and can dress a chamber or carve new ones in short time. It is their ability to shape stone that puts their dwellings in high demand and they often must move on because someone more powerful wants their home.
The largest settlement the PCs encountered was probably Lurumesh (although you can create others if your PC background sent you far afield). This city has about 4000 people, mostly dwaro with a drow and a tiefling community plus a handful of surfacers and humans.
Most other settlements are fairly small- 40 to 400 people. Some example ones PCs may have encountered:
Kaliantroc A tiefling community of 300 led by the mystic Conteeze, a long lived human-tiefling who came from the surface a 100 years ago.
Setriss A mixed settlement led by Hogess, a dwaro trader who runs the place as a barter town. Hogess uses tiefling enforcers and feeds those who break deals to a tame drake of prodigious size. The drake eats a lot so Hogess keeps very strict rules on deals. Despite the danger, it is conveniently located and as Hogess charges nothing for trade, a popular spot. (He makes his money cherry picking trade goods and a brothel).
There are all manner of creatures in the underworld. Demons are common and sometimes create their own settlements but these are almost always closed to non-demons. Otherwise, you can find dragons, spiders, giant insects, umber hulks and any other typical underground creatures. Feel free to work whatever you like into you backgrounds.
There are various organic seeps from the surface as well as magical seeps that provide food for the underworld. This is basically organic sludge, often an odiferous mud, sometimes something more palatable in the case of the magic sources. Sentient beings rarely consume this matter directly and instead let a whole eco-system of underworld creatures process it first.
At the bottom are molds, various worms and insects that eat the sludge. These can be eaten when desperate for food (and all of your PCs were probably desperate at one point or another) but these in turn are eaten by more palatable food sources ranging from rats (some up to human size), various giant insects (taste like shrimp), and Luns (a type of pig, smaller than surface variants). Of course, many creatures are not very picky and will consume any creature that crosses their path, like adventurers.
Many underworld creatures can go dormant, similar to hibernation when food is scarce.
Some of the sludge molds produce a faint light, even when the sludge has dried up for a time or when tracked into other places. This is much dimmer than a candle but still enough to get by in many places. There are also geological formation that provide somewhat stronger light, crystals that have trapped enough magic energy to serve as light, and so on.
Settlements and cities are generally lit by magic, even when the residents have darkvision.
PCs may have acquired low or dark vision during their stay (See see Paragon Leveling).
Shadows and Plots
The more perceptive of you may have picked up the following undercurrents:
Many dwaro believe that the being that destroyed their Empire thousands of years ago and created the taint that has led to demons was the Titan himself.
The giant with the spear that you saw the day you killed the shadow dragon matriarch has been seen before although not always giant sized. He seems to be stirring up the dragons, even changing them. Shadow dragons, for instance, were never known of before he showed up.
There are other travellers than the giant who walk the tunnels. Some seem to seek epic battles, others seem to be seeking knowledge or have hidden purposes. Many of you were mistaken for such travellers in your 4 years below ground.