This is the biggest quest for the Brecilian Forest. Doing it can net a full set of juggernaught armor, which is very good general armor that has a strong resistance to magic. It's actually one of the most useful pieces of equipment against the archdemon at the end, so you definitely want it.
The first three pieces are fairly straightforward to get, but not necessarily easy. There are three ancient tombstones in the forest. The first one you'll see is in the West Brecilian Forest in a spot that looks down on the Grand Oak. You can't do much with this one though. You will normally have to break the wards on the one in the East Brecilian Forest first.
The second one is in the East Brecilian Forest. It's in the back corner and to the left in what looks like a ruined temple. Save and then examine the tombstone. Once you disturb its wards, a revenant will spawn with a few skeleton soldiers. Revenants are really hard to kill, especially at a low level.
There isn't much of a trick though. A tank need to distract the revenant, a rogue needs to backstab him or at least dish out a little extra damage, and a mage needs to work crowd control on the skeletons along with the warrior. The biggest problem for a usual combat group is that the revenant can swing backwards. This means your rogue can easily take a big hit at any point in the fight. You really need to watch their health.
Also, make sure you look for skeleton mages. There can occasionally be a mage in the group and they are top priority. Make sure a warrior or a rogue rushes them before they can do too much damage.
With a little luck, the revenant will die after a bit of a grind.
You should now be able to go back to the first tombstone we passed. It's marked on the map with an “X", in case you have trouble finding it again.
Disturb the wards and kill the revenant here.
The third one is on the way into the Lair of the Werewolves. It's just past the fog and in a ruined site. Once again, just disturb the tombstone and then take it out.