Void Crystal

Void Crystal Farming
Professions Req. : Enchanter, Tailor (or friends with these)

If you need several of these (like for Mongoose http://www.wowhead.com/?spell=27984 ) and do not want to spend 62 gold per Void Crystal or ninja epic items to DE, then a good Tailoring alternative is to craft (or have a tailor friend craft) Soulcloth Gloves http://www.wowhead.com/?item=21863 for the soul purpose of disenchanting them for the Void Crystal. (They do not sell for enough to justify selling them and buying Voids on my server.)

Soulcloth Gloves require:
5 Bolts of Soulcloth (5 Bolts takes 30 Netherweave [7.5 g] and 40 Soul Essences [62g]) http://www.wowhead.com/?item=21844
6 Knothide Leather [4.3g] http://www.wowhead.com/?item=21887
4 Rune Thread [2g]

According to WoWHead.com's estimated Auction House buy out prices (which I calculated and listed above in brakets) the final cost would be about 75.8 gold to craft this, if you bought all of the mats.

HOWEVER: Netherweave is extremely easy to farm for or get from guildies; same with knothide; Soul Essences drop ALOT in Karazhan, and tailors are the only ones who can use them...most nice guildies are willing to give them to you for free or at a fraction of the AH price.
I would say the true cost of crafting the gloves is about 35-40 gold (buying about half the mats). Even cheaper if you farm mats and have a Karazhan raiding guild with good friends in it. If you have access to lots of Essences, its pretty much just a solo farming exercise.

After that, just DE them for a Void Crystal (and maybe even 2 Crystals). Repeat this little process untill you have as many as you need.

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Khthon
Level 70 Druid
Jaedenar, US Server
http://armory.worldofwarcraft.com/#character-sheet.xml?r=Jaedenar&n=Khthon

Found by disenchanting expansion epics. Basically, it's the nexus crystal of TBC.

And like the nexus you can get lucky and get them from rare stuff. Got it from Helm of Desolation.