Bolt of Soulcloth

Bolts of Soulcloth are made by tailors by infusing Netherweave with the essence of souls found in Karazhan. They are used in items with high arcane resistance.

Unlike other fabrics, Soulcloth only exists as bolts, and not as individual pieces of cloth.

It is used in the production of the Soulcloth Embrace set, which is moderate-arcane-resistance, multi-purpose gear designed for fights against arcane bosses where the player does not want to reduce their stats excessively.

Source
This pattern can be learned by Tailors with a skill level of 345, from Pattern: Bolt of Soulcloth.

The pattern is sold in limited supply by the following vendors:
 * at Telaar in Nagrand
 * at Garadar in Nagrand
 * at The Bazaar in Silvermoon City
 * at Traders' Tier in The Exodar

Tailoring

 * Soulcloth Gloves
 * Soulcloth Shoulders
 * Soulcloth Vest

Leatherworking

 * Cloak of Darkness
 * Shadowprowler's Chestguard