These are hard to find - but if you get em used, you can pick em up for as low as .25 cents a piece. They hail from the early 80's and were too "Adult" "dark and "sexual" for Del Rey to touch them at the time.
The Dark Border 1: The Lost Prince
The Dark Border 2: King Chondos' Ride
They are both by Paul Edwin Zimmer. More info
here
Upshot of setting: Along a border of shadow, the evil taint spreads across the continent of the free peoples, held back only by the bordermen warriors and their nearly immortal allies, the Children of Hastur.
Behind the advancing border are ancient ruins of human cities, wreathed in darkness and death, fallen to the Shadow. The Shadow is a land in perpetual twilight and is populated with all manner of monsters, undead and demons of unspeakable power.
The Shadow is True Evil. It cannot be bargained with. It will accept nothing less than the destruction of all life.
The humans are trying to hold on to what they've got. In some areas, to regain their land. The land taken over by the shadow is destroyed, the soil lifeless and grey.
The Dark Border stands on the horizon like a black curtain drawn against the sunlit sky.
There is lots of high power on the good guys' side as well with the Children of Hastur. Ancient artifiacts of power lie in the ruins behind the border, just waiting to be reclaimed. Those on the border know the tales of many of these great weapons and artificats and where they were lost against the shadow. But to retrieve them you must ride into the shadow - find them - and get out alive. No easy task.
This is an excuse to throw magic items, dark ruins/dungeon delving and *endless* bad guys of increasing power at the humans. There is never a need for really complicated hooks here. There is no need to explain why there is a monster like this behind the Border. It's there because it is preparing to attack the party or the borderlands. THAT is why it's there. If you want to start rumaging in the MM1, MM2, MM3, Fiend Folio. Libris Mortis or other D20 products for increasingly weird and fearsome monsters - this setting is the perfect foil for that.
There is lots of infighting, shades of gray and politics on the human side of the border - but the Enemy is the Enemy.
The setting in two other novels (not recommended, but they are okay too) confirms the presence of elves and dwarves in this world as well - so iconic races are suitable if you want to use it.
The books include a map of the Borderlands.
The author was the brother of Marrion Zimmer Bradley. Regrettably, Paul E. Zimmer passed away - but the books are well written and the fighting is particularly well described. These books are diamonds in the rough.
Look for them used or on Amazon. They have bluish purplish covers.
