NOTE: Since this cave has many difficult creatures, and the rewards are not terribly exciting, it is highly reccommended that you get a team together and complete all of the quests in this cave at one time, instead of making multiple trips.
Go down into the Edron Orc Cave. The entrance is southwest of the castle (here). You can access this area through a drain in the basement of the Edron Flats.
Once in the hole, you will meet only Poison Spiders, Orcs, Orc Spearmen and Orc Warriors. Follow the cave to the west and then south. There are a couple of side rooms, but basically there is only one way to go.
As soon as you drop down this hole, you will be surrounded by Orc Shamans. Remember, energy attacks do not work on shamans, so use GFBs to clear the room (the summoned snakes make it hard to aim, and your knights can't get through them to attack the Shamans).
On this level, go north, facing more Orc Shamans and one or two Orc Berserkers. You will walk over a tile-floor area - this means you are on the right track.
Just go down the hole and kill the Giant Spider. You can find the Dark Armor inside a dead body on the ground. Instead of killing the GS you can also put on defensive mode, run to the body, grab the loot and go back.