Welcome here also in our forum.
I don't know the Almanac, so I don't know what's in there about the Shadowlands, but in the Core Rulebook are indeed only a few sentences about the Shadowlands.
I'm in a hurry right now, but hopefully someone else has some more words.
The Shadowlands are former parts of the Middenrealm. In former years a dark magician and half-god called Borbarad conquered the parts. After his defeat in 1028 BF the country was parted among his greatest followers, the so called "heptarchs".
In which year do you plan to start with the campaign? The Shadowlands changed during the years a little bit, who is heptarch and which part belongs to which heptarch.
Actually, meanwhile the Shadowlands are back in the Middenrealm, since Queen Rohaja lead a military campaign and other events (which can all be played in an adventure campaign).
There is a kind of war between the Middenrealm and the Shadowlands.
The Shadowlands are a unique country, with undeads, perverted land, and some unique religion. The heptarchs are of different colour. This makes every part of the Shadowlands a little bit different to the others.
Achaz are unique and rather ancient race way past their pinnacle of civilization. They are not human, not in thinking, behavior and other things. They can not stand cold temperatures, which makes them hard to play in northern countries during the winter. The Tulamids and Novadis don't like them, which makes them hard to play in their countrys. They are strangers every where because of these differences, and not just in their appearance.