This is prolly getting nerfed soon but it still works at this moment.
Go to areas scourge is invading. there are usually 3/4 or so camps with mobs and a crystal, mobs respawn until crystal is 0 hp and then there are 4 shadow of doom's. All mobs have about 30% droprate of a necrotic runestone thingy. The shadows of doom aloways drop 30, rare mobs often drop 3 as well as some epics.
what I do, depending on how many people are busy, is preferably pull a camp, aoe, loot, run to other camp, pull, aoe, loot, run back to camp 1, pull aoe and loot, etc, until one stone is almost down, stick to that area and do the shadows one by one.
This night, I farmed these for about 3 hours. In total I got 10 or so epics, and 750 necrotic runestones. At the vendor in lights hope you can buy epics for 15 of these stones. I bought around 50 epics.
I disenchanted the total of 60 epics I gained in 3 hours of grinding and had around 100 void shards. (they seem to have a high 2x drop rate).
So that's 100 void shards in 3 hours. these I broke down into large prismatic shards, (make more money on my server.) lps's go for 10-12g a piece on my server. I spammed trade, mass selling them for 7g a piece. And made 1400 gold in 3 hours of grinding and one hour of selling.
1400 gold, four hours. D:
As I said tho, blizz will prolly nerf the disenchantable items form the vendor, like they have done in the past too. However, for now it does still work. Use it while you can!
PPS: the longer you wait, the more likely people will catch on, the more likely prices on your shards drop too. So, you have to sell the shards low and in large quentities. It will flood the market, prices will decrease afterwards. Or keep the shards until when prices gone up again. :P
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