As to the Barrowfields, however, there I can be of (minor) help. The FRCS, Volo's Guide To the Dalelands (the most extensive entry: you don’t need the other two if you have this one), and The Dalelands 2nd Edition accessory all mention the Barrowfields, but let's recap: about thirty miles east of Peldan's Helm is a large, grassy glen (in this case, a broad stream valley or basin whose stream has 'gone under,' leaving no visible surface water) in which "a dozen or more" old mounds stand. Mists from the River Ashaba often cling to them, and (of course) they’re said to be haunted. There are actually sixteen grass-covered mounds, each about two hundred feet long and rising about twenty feet from the surrounding earth with fairly gentle side-slopes and more abrupt end-slopes. They all run in the same north-south direction, parallel to each other, like glacial drumlins.
If adventurers visit the barrows by day, they'll be aware of nothing more than an unpleasant feeling of being watched. If they try 'prying magics,' or sleep near the barrows, their minds will be invaded by disturbing visions of silently menacing robed watchers -- who if confronted will prove to be wraiths with skull-heads that melt away when revealed.
If any of the barrows (all of which lack visible doors, though many have grassed-over pits in their sides from early diggings) are dug into or blasted open, skeletons will be revealed (in 3rd Edition, "Human Warrior Skeleton" undead, about ten percent having odd magical abilities such as: split into two intact skeletons if touched by a spell; able to blink about to attack, able to deliver various touch-attack magics, and so on). These will all attack fearlessly and tirelessly, pursuing all living creatures to the edge of the glen or until destroyed. "Slain" skeletons will crumble into dust. If you introduce a Shade necromancer, of course, these skeletons would become perfectly obedient troops under the necromancer's command.
The main 'monsters' of the barrows are wraith-like undead Netherese who are linked to specific magic items (mainly wands and scepters) buried in the barrows -- if the items are carried off, the wraiths (which can't be turned) go with them. If a wraith is destroyed, it vanishes back into the magic item, only to emerge some days later and attack again. (Adventurers can wield the item and call on its powers, but don't gain any measure of control over the wraiths linked to it.) Items with wraiths "inside" them become more difficult to destroy, but breaking such an item releases the wraiths in a VERY powerful, item-is-ground-zero explosion of withering unlife. In 3rd Edition terms, I'd make these dread wraiths, except that their 'spawn' rise instantly as controlled zombies, not wraithlike creatures, they aren't harmed in any way by sunlight, and they can't be turned, rebuked, commanded, or bolstered (they can be 'destroyed' in battle, but not disrupted -- except by breaking the item they're linked to, which destroys them in the explosion I referred to earlier).
Hidden in the heart of some of the barrows are whatever variants of powerful liches you want to introduce into your campaign. In the 'home' Realms campaign, one of the inadvertently-freed inhabitants of the Barrowfields was a "flying skull" type of lich that lurked unseen, as much as possible, observing the living and manipulating individuals (often wizards of low level) by means of silently-cast spells into doing things it wanted done or even becoming thralls who served it for years. This entity became a long-term behind-the-scenes foe of the Knights just because they were present in the Dales as do-gooders, and it wanted to expand its influence across the Dales, ruler by ruler, without hindrance.
There is a 'dungeon' of sorts linking two of the barrows (a single-level labyrinth of burial chambers and passages), but as the Knights were never foolish enough to delve that far, my notes on it are safely packed up and lost in the infamous Basement Boxes. So have fun putting whatever you want down there. An ancient portal to somewhere interesting in Faerun would be fun.
If you have access to the EPIC LEVEL HANDBOOK, a Worm That Walks can 'stand in' for the unique undead mage I had lurking near the Barrowfields in the 'home' Realms campaign. This fell creature, Halamorthaun, came to be in the battles that laid waste to Cormanthyr (though it lay dormant and unnoticed for centuries) and during the present day lurks near the barrows, observing who visits and revealing itself in attacks only on those it judges weak and isolated enough to destroy without being seen by others.
Heh-heh. Enjoy. :}